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          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations"/>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ArrangementsAndNonarrangementTransactionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaborative Arrangement and Arrangement Other than Collaborative</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentTransitionReport_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Transition Report</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentTransitionReport_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Transition Report</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClinicalTrialMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Clinical trial.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClinicalTrialMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Clinical Trial [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClinicalTrialMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Clinical Trial [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClassOfWarrantOrRightIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Class of warrant or right issued.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClassOfWarrantOrRightIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class Of Warrant Or Right Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ClassOfWarrantOrRightIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrant issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_StatementGeographicalAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Geographical [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_StatementGeographicalAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Geographical</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleSecuritiesDebtSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Debt Securities, Available-for-Sale, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Prepaid Expense and Other Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Prepaid expenses and other assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedStockUnitsRSUMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Restricted Stock Units (RSUs) [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedStockUnitsRSUMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Non-Vested Restricted Stock Units (RSUs) [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedStockUnitsRSUMember_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Restricted Stock Units [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Inputs, Level 1 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Quoted Prices (Unadjusted) in Active Markets Level 1 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue from Contract with Customer, Excluding Assessed Tax</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Total revenues</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total revenues</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantRisksAndUncertaintiesPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Significant Risks and Uncertainties</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantRisksAndUncertaintiesPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Significant Risks And Uncertainties Policy [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantRisksAndUncertaintiesPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Disclosure of accounting policy for Significant risks and uncertainties.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdditionalDefaultInterestRate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Additional default interest rate.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdditionalDefaultInterestRate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Additional Default Interest Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdditionalDefaultInterestRate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Default interest rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unrecognized compensation expense related to stock option and RSUs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Amount, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OfferingPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Offering price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OfferingPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Offering Price Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OfferingPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Offering price per share.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Title of Individual [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_TitleOfIndividualWithRelationshipToEntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Title of Individual</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsToBeReturnedUponTerminationOfAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Upfront payments to be returned upon termination of agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsToBeReturnedUponTerminationOfAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Upfront Payments To Be Returned Upon Termination Of Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsToBeReturnedUponTerminationOfAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Upfront payments returned upon termination of agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research and Development [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UcbLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">UCB license agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UcbLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Ucb License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UcbLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">UCB License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Fair Value, Assets and Liabilities Measured on Recurring Basis [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfFairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule of Fair Values of Financial Assets and Liabilities Measured at Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease, Liability, Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Right-of-use liability, less current portion</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockByClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Stock by Class [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockByClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Stock By Class [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueDisclosureItemAmountsDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueDisclosureItemAmountsDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Measurement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Securities Available For Sale [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Reimbursable costs.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Reimbursable Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reimbursable costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities, Fair Value Disclosure</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class of Stock [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Class Of Stock [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfStockLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Class of Stock [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherReceivablesNetCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Receivables, Net, Current [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaymentFee_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepayment Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaymentFee_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Prepayment Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForFees_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Pre-Payment Fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForFees_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments for Other Fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities and Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total liabilities and stockholders&#8217; equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressStateOrProvince_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, State or Province</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressStateOrProvince_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, State or Province</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Value, New Issues</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock issued at fair value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based compensation, stock options outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Outstanding, Number, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted average grant date fair value of options</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Comprehensive Income (Loss), Net of Tax, Attributable to Parent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Comprehensive loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_TradingSymbol_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trading Symbol</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_TradingSymbol_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trading Symbol</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Shares, Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Available For Sale Securities [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period, Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodTotalFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted average grant date fair value per share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid Expense and Other Assets, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid expenses and other current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total prepaid expenses and other current assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income (Loss) Attributable to Parent [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Numerator&#8212;basic and diluted:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RemainingContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Remaining contractual obligation.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RemainingContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Remaining Contractual Obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RemainingContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Remaining contractual obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Long-Term Debt Instruments [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Shares, Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Beginning balance, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Ending balance, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Capital Shares Reserved for Future Issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Shares available for issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of prepayment premium of principal amount outstanding thereafter.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of Prepayment Premium Of Principal Amount Outstanding Thereafter</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of prepayment premium of principal amount outstanding thereafter prior to maturity date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated Other Comprehensive(Loss) / Income [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">AOCI Attributable to Parent [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndTwentyTwoPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand and twenty two performance awards.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndTwentyTwoPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand And Twenty Two Performance Awards [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndTwentyTwoPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2022 Performance Awards [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Upfront payments received.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Upfront Payments Received</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License agreement upfront payment received</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableExpenseDueFromCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Reimbursable expense due from counterparty.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableExpenseDueFromCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Reimbursable Expense Due From Counterparty</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ReimbursableExpenseDueFromCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Reimbursable expense due from counterparty</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressCityOrTown_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, City or Town</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressCityOrTown_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, City or Town</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class of Warrant or Right, Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrants Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenantWaived_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Line of credit facility minimum cash covenant waived.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenantWaived_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line Of Credit Facility Minimum Cash Covenant Waived</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenantWaived_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Minimum cash covenant waived</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Loan Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of shares, Vested and exercisable, end of the period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Exercisable, Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Additional Paid-in Capital [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Additional Paid-In Capital [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonUsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Non-US [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonUsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Ex-U.S. Countries [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Employee service share based compensation recognized stock options not met.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Service Share Based Compensation Recognized Stock Options Not Met</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recognized compensation expense related to stock option not met</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities, Current [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current liabilities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets, Current [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsCurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current assets:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_CounterpartyNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Counterparty Name [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_CounterpartyNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Counterparty Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock, Par or Stated Value Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, par value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease, Liability, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseLiabilityCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current portion of right-of-use liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Lender Name [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Lender Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Property, Plant and Equipment, Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Property and equipment, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Property, Plant and Equipment, Net, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CurrentPortionOfCapitalLease_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Current portion of capital lease.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CurrentPortionOfCapitalLease_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Current Portion Of Capital Lease</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CurrentPortionOfCapitalLease_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current portion of capital lease</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentIncomeInterest_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Investment Income, Interest</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InvestmentIncomeInterest_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest income</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BusinessCombinationContingentConsiderationLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Business Combination, Contingent Consideration, Liability, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BusinessCombinationContingentConsiderationLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Business Combination, Contingent Consideration, Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BusinessCombinationContingentConsiderationLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Contingent liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ScenarioUnspecifiedDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ScenarioUnspecifiedDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Scenario</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaborative Arrangement and Arrangement Other than Collaborative</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCentralIndexKey_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Central Index Key</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCentralIndexKey_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Central Index Key</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Class of Warrant or Right, Number of Securities Called by Warrants or Rights</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrants issued (in shares)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Plan Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Plan Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand seven and two thousand fifteen stock incentive plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand Seven And Two Thousand Fifteen Stock Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2015 and 2007 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasuryAndGovernmentMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">US Treasury and Government [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasuryAndGovernmentMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">US Treasury and Federal bonds [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedCashNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Restricted Cash, Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RestrictedCashNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Restricted cash</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets, Fair Value Disclosure</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock, Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, shares outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Preferred Stock, Shares Outstanding, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Preferred Stock, Shares Outstanding, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total current liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityTaxIdentificationNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Tax Identification Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityTaxIdentificationNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Tax Identification Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Operating Capital [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Changes in operating assets and liabilities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Value, Employee Stock Purchase Plan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock purchase under ESPP</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedArrangementsToObtainGoodsAndServicesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Plan Name [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PlanNameAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Plan Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_SegmentGeographicalDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Geographical [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_SegmentGeographicalDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Geographical</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Assets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Assets_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Lease, Right-of-Use Asset</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingLeaseRightOfUseAsset_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Right-of-use asset, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Nonvested Award, Option, Cost Not yet Recognized, Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedStockOptions_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share based compensation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Accrued Liabilities and Other Operating Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued expenses and other liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityRegistrantName_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Registrant Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityRegistrantName_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Registrant Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Debt instrument tranche two</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Tranche Two [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Tranche 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine3_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, Address Line Three</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine3_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Address Line Three</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Retained Earnings (Accumulated Deficit)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated deficit</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Retained Earnings (Accumulated Deficit), Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromStockPlans_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from Stock Plans</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromStockPlans_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from Employee Stock Purchase Plan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CapitalLeaseLessCurrentPortion_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Capital lease, less current portion.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CapitalLeaseLessCurrentPortion_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Capital Lease Less Current Portion</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CapitalLeaseLessCurrentPortion_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Capital lease, less current portion</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Retained Earnings [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RetainedEarningsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated Deficit [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Number of Shares, Unvested, Ending balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Number of Shares, Unvested, Beginning balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Shares, Employee Stock Purchase Plans</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock purchase under ESPP, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MinimumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Minimum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MinimumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Minimum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from Stock Options Exercised</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from stock option exercises</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Significant Accounting Policies [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Significant Accounting Policies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule of Prepaid Expenses and Other Current Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_HerculesCapitalIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Hercules Capital, Inc.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_HerculesCapitalIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Hercules Capital Inc [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_HerculesCapitalIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Hercules [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ScenarioForecastMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Forecast [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_ScenarioForecastMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Scenario Forecast [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityComponentDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity Component [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityComponentDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Equity Component</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeStockOptionMember_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Stock options [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Adjustments to reconcile net loss to net cash used in operating activities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income (Loss) Available to Common Stockholders, Diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss attributable to common stockholders - diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net loss attributable to common stockholders - diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument, Basis Spread on Variable Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basis spread on variable rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCurrentReportingStatus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Current Reporting Status</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCurrentReportingStatus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Current Reporting Status</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Shares Authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research and Development Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research and development</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Research and development expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Research and Development Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RevenueRecognizedUponTransferOfLicenseAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Revenue recognized upon transfer of license agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RevenueRecognizedUponTransferOfLicenseAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue Recognized Upon Transfer Of License Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RevenueRecognizedUponTransferOfLicenseAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Revenue recognized upon transfer of license agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Share based compensation arrangement by share based payment award stock option grant date fair value.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Stock Option Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Options grant date fair value amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Assets [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AssetsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">ASSETS</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from issuance of common stock in follow-on public stock offering, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from issuance of common stock in at-the-market stock offering, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Proceeds from issuance of common stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCorporationMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Incyte corporation.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCorporationMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Incyte Corporation [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCorporationMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Incyte Corporation [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock reserved for issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Number of Shares Available for Grant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidConferenceMeetingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid conference, meeting costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidConferenceMeetingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid Conference, Meeting Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidConferenceMeetingCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Prepaid conference, meeting costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestReceivableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Receivable, Current</link:label>
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          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Value, Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableAccruedLiabilitiesAndOtherLiabilitiesDisclosureCurrentTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounts Payable, Accrued Liabilities, and Other Liabilities Disclosure, Current [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableAccruedLiabilitiesAndOtherLiabilitiesDisclosureCurrentTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued Expenses and Other Current Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPayments_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Upfront payments.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPayments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Upfront Payments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontPayments_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Upfront milestone payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Current Fiscal Year End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Current Fiscal Year End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Noncash Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock-based compensation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensation_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Share-Based Payment Arrangement, Noncash Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Income (Loss)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Loss from operations</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Realized Gain (Loss)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Realized gains or losses recognized on the sale or maturity of available-for-sale securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleRealizedGainLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Debt Securities, Available-for-Sale, Realized Gain (Loss), Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedResearchAndDevelopmentCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Accrued Research And Development Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedResearchAndDevelopmentCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Research And Development Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedResearchAndDevelopmentCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued research and development costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research and Development Expense [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research and Development [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NatureOfOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Nature of Operations [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NatureOfOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Nature of Business</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from sales and maturities of short-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Proceeds from Sale and Maturity of Debt Securities, Available-for-Sale, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock issued during period value at the market offering net.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Value At The Market Offering Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from ATM sales</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Significant agreements disclosure.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Significant Agreements Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Significant Agreements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share based compensation expense, total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RefundableFeePayble_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Refundable fee payble</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RefundableFeePayble_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Refundable Fee Payble</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_RefundableFeePayble_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Refundable Fee Payble</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated other comprehensive (loss) gain</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">AOCI, Debt Securities, Available-for-Sale, Adjustment, after Tax</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Net of Forfeitures</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock option granted to certain executives, consultants and employees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Net of Forfeitures, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Comprehensive Income (Loss), Net of Tax [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other comprehensive loss:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Employees.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employees [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Employees [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FederalBondsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Federal bonds.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FederalBondsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Federal Bonds [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FederalBondsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Federal Bonds [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DepreciationAndRelatedAdjustment_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Depreciation and related adjustment.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DepreciationAndRelatedAdjustment_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Depreciation And Related Adjustment</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DepreciationAndRelatedAdjustment_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Depreciation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByMeasurementBasisAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Measurement Basis [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByMeasurementBasisAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Measurement Basis</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Nonoperating Income (Expense)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Other Nonoperating Income (Expense), Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForProceedsFromDepositOnLoan_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Payment on term loan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForProceedsFromDepositOnLoan_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepayment of loan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsForProceedsFromDepositOnLoan_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments for (Proceeds from) Deposit on Loan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExercisedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number Of Pre Funded Warrants Exercised For Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExercisedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number Of Pre Funded Warrants Exercised For Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExercisedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of pre funded warrants exercised for common stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Expenses [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Operating expenses:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromLicenseFeesReceived_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from License Fees Received</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromLicenseFeesReceived_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtSecuritiesAvailableForSaleTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Available-for-Sale Securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidStateAndLocalTaxes_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Prepaid State and Local Taxes</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidStateAndLocalTaxes_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid State and Local Taxes</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidStateAndLocalTaxes_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid state and local taxes</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments and Contingencies Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Assets, Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Grants in Period, Gross</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based compensation, stock options granted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Accrued Liabilities, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stockholders' Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodSharesAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock issued during period shares at the market offering net.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodSharesAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Shares At The Market Offering Net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodSharesAtTheMarketOfferingNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from ATM sales, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasicLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasicLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Earnings Per Share Basic [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsReceivableAndOtherOperatingAssets_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Other receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsReceivableAndOtherOperatingAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsReceivableAndOtherOperatingAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Accounts Receivable and Other Operating Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Gain, before Tax</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unrealized Gains</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Vested and exercisable, end of the period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Exercisable, Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity, Attributable to Parent [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stockholders&#8217; equity:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line of Credit Facility, Maximum Borrowing Capacity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Aggregate maximum borrowings</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockholdersEquityNoteWarrantsOrRightsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule of Common Stock Reserved for Future Issuance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountingPoliciesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounting Policies [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Cowen and Company LLC.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cowen And Company L L C [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cowen and Company LLC [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidSubscriptions_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Prepaid Subscriptions</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidSubscriptions_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid Subscriptions</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PrepaidSubscriptions_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid subscriptions</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_StatementScenarioAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_StatementScenarioAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Scenario</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based Compensation Arrangement by Share-based Payment Award [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share, Basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss per share attributable to common stockholders&#8212;basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Net loss per share attributable to common stockholders&#8212;basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Earnings Per Share, Basic, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments and Contingencies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingencies_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commitments and contingencies (Note 12)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Income Statement [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commercial Paper, Not Included with Cash and Cash Equivalents [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commercial Paper [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndNineteenPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand and nineteen performance awards.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndNineteenPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand And Nineteen Performance Awards [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndNineteenPerformanceAwardsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2019 Performance Awards [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Vested and Expected to Vest, Outstanding, Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based compensation, stock options vested</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statistical Measurement [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statistical Measurement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Operating Activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net cash used in operating activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentPeriodEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Period End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentPeriodEndDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Period End Date</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statistical Measurement [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statistical Measurement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commitments and Contingencies Disclosure [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommitmentsAndContingenciesDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commitments and Contingencies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedMilestoneCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Accrued milestone costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedMilestoneCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued milestone costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedMilestoneCosts_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Milestone Costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share, Diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss per share attributable to common stockholders&#8212;diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Net loss per share attributable to common stockholders&#8212; diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareDiluted_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Earnings Per Share, Diluted, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GrossProceedsFromOffering_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Gross proceeds from offering</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GrossProceedsFromOffering_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Gross Proceeds From Offering</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GrossProceedsFromOffering_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Gross proceeds from offering.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument, Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument, Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForFullyRefundableFee_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Payment for fully-refundable fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForFullyRefundableFee_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payment for Fully-refundable Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForFullyRefundableFee_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Payment for Fully-refundable Fee</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Derivative liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Derivative Liability, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Hierarchy and NAV [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Hierarchy and NAV</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Related Party Transaction [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Related Party Transaction</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MilestoneExpensesPaid_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Milestone Expenses Paid</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MilestoneExpensesPaid_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Milestone Expenses Paid</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Additional Paid in Capital, Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Additional paid-in capital</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfMaturitiesOfLongTermDebtTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Maturities of Long-Term Debt [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfMaturitiesOfLongTermDebtTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Maturities of Principal Obligations Under Long-term Debt</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasurySecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">US Treasury Securities [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_USTreasurySecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">U.S. Treasury [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Number of shares, Exercised/Released</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2009/role/netLabel">Stock options exercised, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Number of shares, Exercised/Released</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Exercises in Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from exercise of stock options, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities and Equity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">LIABILITIES AND STOCKHOLDERS&#8217; EQUITY</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfEndOfTermChargeToFutureDraws_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of end of term charge to future draws.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfEndOfTermChargeToFutureDraws_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of End Of Term Charge To Future Draws</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfEndOfTermChargeToFutureDraws_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of end of term charge to future draws</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Earnings Per Share, Basic, by Common Class, Including Two Class Method [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Earnings Per Share Basic By Common Class [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ExerciseOfPrefundedWarrantsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Exercise Of Prefunded Warrants, Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ExerciseOfPrefundedWarrantsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Exercise of prefunded warrants shares.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ExerciseOfPrefundedWarrantsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prefunded warrants, exercise, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressPostalZipCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, Postal Zip Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressPostalZipCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Postal Zip Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityInteractiveDataCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Interactive Data Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityInteractiveDataCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Interactive Data Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrivatePlacementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Private Placement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrivatePlacementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Private Placement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase (Decrease) in Accounts Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accounts payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Increase (Decrease) in Accounts Payable, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfRevenueExtensibleList_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenue, Product and Service [Extensible Enumeration]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_TypeOfRevenueExtensibleList_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Type of revenue [extensible list]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CreditFacilityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Credit Facility [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CreditFacilityDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Credit Facility</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of Shares, Granted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Exercised/Released</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangements by Share-Based Payment Award, Options, Exercises in Period, Weighted Average Exercise Price</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Incorporation, State or Country Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Incorporation, State or Country Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">State of incorporation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity Components [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Equity Components</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Term loan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Term Loan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TermLoanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Term Loan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Vested</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LocalPhoneNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Local Phone Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LocalPhoneNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Local Phone Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale of Stock [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementOfCashFlowsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement of Cash Flows [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement and Measurement Inputs, Recurring and Nonrecurring [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Measurements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Organization, Consolidation and Presentation of Financial Statements [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Par or Stated Value Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockParOrStatedValuePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, par value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementsExpirationPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">License agreements expiration period.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementsExpirationPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">License Agreements Expiration Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementsExpirationPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License expiration year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VestingOfRestrictedStockUnitsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Vesting of Restricted Stock Units, Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VestingOfRestrictedStockUnitsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Vesting of Restricted Stock Units, Shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VestingOfRestrictedStockUnitsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vesting of RSU, shares</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument, Interest Rate, Stated Percentage</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentInterestRateStatedPercentage_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Nonvested Award, Cost Not yet Recognized, Period for Recognition</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted average period to recognize compensation expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaboration payable, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaboration Payable, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaboration Payable, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Share-Based Compensation Arrangements by Share-Based Payment Award [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Share Based Compensation Arrangements By Share Based Payment Award [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MaximumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Maximum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_MaximumMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Maximum [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaboration receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaboration receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Collaboration receivable, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaboration receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Award Vesting Rights</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingRights_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vesting period, description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Dividends_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Dividends</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Dividends_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Deemed dividend due to warrant reset</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Dividends_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Dividends, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Related Party Transaction [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RelatedPartyTransactionAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Related Party Transaction</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Scenario two.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario Two [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioTwoMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Available through November 30, 2022 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel3Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Inputs, Level 3 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel3Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Significant Unobservable Inputs Level 3 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">APIC, Share-Based Payment Arrangement, Increase for Cost Recognition</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock-based compensation expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">APIC, Share-Based Payment Arrangement, Increase for Cost Recognition, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Recurring and Nonrecurring [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Research and Development Arrangement, Contract to Perform for Others [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ResearchAndDevelopmentArrangementContractToPerformForOthersLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Research And Development Arrangement Contract To Perform For Others [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">NET INCREASE (DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsPeriodIncreaseDecreaseIncludingExchangeRateEffect_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Including Exchange Rate Effect</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Debt instrument, tranche three.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument Tranche Three [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DebtInstrumentTrancheThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Tranche 3 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PayablesAndAccrualsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payables and Accruals [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MilestonePaymentDue_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Milestone payment due.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MilestonePaymentDue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Milestone Payment Due</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MilestonePaymentDue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Milestone payment payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FirstAmendmentToLoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">First amendment to loan and security agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FirstAmendmentToLoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">First Amendment To Loan And Security Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FirstAmendmentToLoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">First Amendment to Loan Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Forfeitures in Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share-based compensation, stock options cancelled</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">License agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Potential Dilutive Securities Excluded from Computation of Diluted Net Loss per Common Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LiabilityRelatedToLetterAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Liability related to letter agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LiabilityRelatedToLetterAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liability Related To Letter Agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LiabilityRelatedToLetterAgreement_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Liability related to letter agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CityAreaCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">City Area Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CityAreaCode_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">City Area Code</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Operating Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">CASH FLOWS FROM OPERATING ACTIVITIES:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Selling, general and administrative</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">General and Administrative Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">General and Administrative Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpensesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Selling, General and Administrative [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SellingGeneralAndAdministrativeExpensesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Selling, General and Administrative Expenses [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number of Shares Outstanding, Basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted-average number of common shares used to compute net loss per share attributable to common stockholders &#8212;basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Weighted-average number of common shares used to compute net loss per share attributable to common stockholders&#8212;basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Weighted Average Number of Shares Outstanding, Basic, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationExpenseDueToCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaboration expense due to counterparty.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationExpenseDueToCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaboration Expense Due To Counterparty</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationExpenseDueToCounterparty_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaboration expense due to counterparty</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Options, Nonvested Options Forfeited, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsNonvestedOptionsForfeitedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair value of restricted stock units vested</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DevelopmentCostsAssociatedWithClinicalTrials_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Development costs associated with clinical trials.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DevelopmentCostsAssociatedWithClinicalTrials_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Development Costs Associated With Clinical Trials</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DevelopmentCostsAssociatedWithClinicalTrials_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Development costs associated with clinical trials</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Financing Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">CASH FLOWS FROM FINANCING ACTIVITIES:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Restricted Stock Unit, Activity [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Restricted Stock Units Activity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statement [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalPeriodFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Fiscal Period Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalPeriodFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Fiscal Period Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities, Noncurrent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesNoncurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total long-term liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccruedProfessionalFeesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Professional Fees, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccruedProfessionalFeesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued professional fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Statement [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities Excluded from Computation of Earnings Per Share, Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Anti-dilutive securities excluded from computation of diluted net loss per common share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Ending Balance 6/30/22</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Beginning Balance 12/31/21</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Warrant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrant [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Asset Class [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Asset Class</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeStockPurchasePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Employee stock purchase plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeStockPurchasePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Stock Purchase Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EmployeeStockPurchasePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Employee Stock Purchase Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash and Cash Equivalents, at Carrying Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash and cash equivalents</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsAtCarryingValue_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Cash and Cash Equivalents, at Carrying Value, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ContractualObligationsTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Contractual obligations term.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ContractualObligationsTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contractual Obligations Term</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ContractualObligationsTerm_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Period of contractual obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Share Based Compensation Expense Related to Issuance of Stock Option Awards to Employees and Non Employees Related to Employee Stock Purchase Plan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Assets, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Change In Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Change In Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Change in fair value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Change in fair value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accretion (Amortization) of Discounts and Premiums, Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Accretion of investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary in Compensation Expense by Type of Award</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disclosure of Share-Based Compensation Arrangements by Share-Based Payment Award [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Common Stock, Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityCommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Entity Common Stock, Shares Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenant_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Line of credit facility minimum cash covenant.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenant_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line Of Credit Facility Minimum Cash Covenant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LineOfCreditFacilityMinimumCashCovenant_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Minimum cash covenant</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities, Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Antidilutive Securities, Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DepositsAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Deposits Assets, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DepositsAssetsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Short-term deposits</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_CoverAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cover [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalYearFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Fiscal Year Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentFiscalYearFocus_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Fiscal Year Focus</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale of Stock [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Sale of Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Grants in Period, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Granted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Potential milestone payments upon achievement of milestone.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Potential Milestone Payments Upon Achievement Of Milestone</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Potential milestone payments to be made</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Scenario Three.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario Three [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioThreeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Not to Draw First from Tranche 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_SecurityExchangeName_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Security Exchange Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_SecurityExchangeName_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Security Exchange Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Long-Term Investments, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Long-Term Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LongTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Long-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">New Accounting Pronouncements, Policy [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Recently Issued and Adopted Accounting Pronouncements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock, Shares Authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockSharesAuthorized_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, shares authorized</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LegalEntityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Legal Entity [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_LegalEntityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Legal Entity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number of Shares Outstanding, Diluted [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Denominator&#8212;basic and diluted:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DevelopmentAndRegulaotryMilestonesExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Development and regulaotry milestones Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_DevelopmentAndRegulaotryMilestonesExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Development and regulaotry milestones Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period, Value, Stock Options Exercised</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from exercise of stock options</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityEmergingGrowthCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Emerging Growth Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityEmergingGrowthCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Emerging Growth Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_AmendmentFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Amendment Flag</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_AmendmentFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Amendment Flag</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Accumulated Gross Unrealized Loss, before Tax</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Unrealized Losses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of prepayment premium of principal amount outstanding in second year.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of Prepayment Premium Of Principal Amount Outstanding In Second Year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of prepayment premium of principal amount outstanding in second year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Basis of Accounting, Policy [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BasisOfAccountingPolicyPolicyTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Significant Accounting Policies</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherCurrentAssetsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Current Assets [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherCurrentAssetsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid Expenses and Other Current Assets</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand fifteen omnibus incentive plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand Fifteen Omnibus Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2015 Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UCBBiopharmaMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">UCB Biopharma.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UCBBiopharmaMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">U C B Biopharma [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UCBBiopharmaMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">UCB Biopharma [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Variable Rate [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Variable Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFileNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Securities Act File Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFileNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity File Number</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Upfront license fees paid.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Upfront License Fees Paid</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Up-front license fee paid</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Vitae Pharmaceuticals, Inc.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Vitae Pharmaceuticals Inc [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Vitae Pharmaceuticals Inc [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCollaborationAgreementAndSharePurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Incyte collaboration agreement and share purchase agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCollaborationAgreementAndSharePurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Incyte Collaboration Agreement And Share Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncyteCollaborationAgreementAndSharePurchaseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Incyte Collaboration Agreement and Share Purchase Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number of pre-funded warrants exchanged for common stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number Of Pre Funded Warrants Exchanged For Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Pre-funded warrants exchange</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Unrealized (loss) gain on short-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">OCI, Debt Securities, Available-for-Sale, Unrealized Holding Gain (Loss), before Adjustment, after Tax</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Unrealized gains on short-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_BayerPharmaAgMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Bayer Pharma AG .</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_BayerPharmaAgMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Bayer Pharma Ag [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_BayerPharmaAgMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Bayer Pharma AG [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NoncashOperatingLeaseExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Non-cash operating lease expense.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NoncashOperatingLeaseExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Noncash Operating Lease Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NoncashOperatingLeaseExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Non-cash operating lease expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketStockOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">At the market stock offering.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketStockOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">At The Market Stock Offering [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketStockOfferingMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">At-the-Market Offering [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accounts Payable, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accounts payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccountsPayableCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Accounts Payable, Current, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Instrument [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DebtInstrumentAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Debt Instrument</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Unvested, Beginning balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Weighted Average Grant Date Fair Value, Unvested, Beginning balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Weighted Average Grant Date Fair Value, Unvested, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Operating Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OperatingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total operating expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedClinicalCostsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Accrued clinical costs current.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedClinicalCostsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Clinical Costs Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedClinicalCostsCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued clinical study and trial costs</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Liabilities and Other Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued expenses and other current liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total accrued expenses and other current liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine2_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, Address Line Two</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine2_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Address Line Two</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Preferred stock, $0.001 par value, 10,000,000 shares authorized; 0 shares outstanding at March 31, 2024 and December 31, 2023</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PreferredStockValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Preferred Stock, Value, Issued</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntitySmallBusiness_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Small Business</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntitySmallBusiness_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Small Business</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityShellCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Shell Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityShellCompany_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Shell Company</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NumberOfOperatingSegments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number of Operating Segments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NumberOfOperatingSegments_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Number of operating segments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_TitleOfIndividualAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Title of Individual [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_TitleOfIndividualAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Title of Individual</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two thousand and seven stock incentive plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand And Seven Stock Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2007 Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesNoncurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities, Noncurrent [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LiabilitiesNoncurrentAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Long-term liabilities:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net Loss per Share Attributable to Common Stockholders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForEndOfTermFees_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">End-of Term fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForEndOfTermFees_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payment for end-of term fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PaymentForEndOfTermFees_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Payment for end-of term fees</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenuesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Revenues [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_RevenuesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Revenue:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AllerganLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Allergan license agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AllerganLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Allergan License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AllerganLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Allergan License Agreement Member</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address, Address Line One</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityAddressAddressLine1_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Address, Address Line One</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash and Cash Equivalents, Fair Value Disclosure</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CashAndCashEquivalentsFairValueDisclosure_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash and cash equivalents</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Contractual Obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Aggregate payment obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Contractual Obligation, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Antidilutive Securities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaboration receivable, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Collaboration Receivable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Increase decrease in collaboration receivable.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Accrued contractual obligation.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Accrued Contractual Obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AccruedContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued contractual obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidRent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid rent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidRent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid Rent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Loan and security agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Loan And Security Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_LoanAndSecurityAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Loan Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementLocationAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Income Statement Location [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementLocationAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Income Statement Location</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProfitLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income (Loss), Including Portion Attributable to Noncontrolling Interest</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProfitLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProfitLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net Income (Loss), Including Portion Attributable to Noncontrolling Interest, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsCashlessExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Number of pre funded warrants cashless exchanged for common stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsCashlessExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Number Of Pre Funded Warrants Cashless Exchanged For Common Stock</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NumberOfPreFundedWarrantsCashlessExchangedForCommonStock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Pre-funded warrants cashless exchange</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidInsurance_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prepaid Insurance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrepaidInsurance_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prepaid insurance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line of Credit Facility, Lender [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityLenderDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Line of Credit Facility, Lender</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OtherReceivablesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other Receivables</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OtherReceivablesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Receivables [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_OtherReceivablesTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Other Receivables [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value, Inputs, Level 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Significant Other Observable Inputs Level 2 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangement and Arrangement Other than Collaborative [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Collaborative Arrangements And Noncollaborative Arrangement Transactions [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentOfWarrantsGrantedForServices_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Prefunded warrants, exercise</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AdjustmentOfWarrantsGrantedForServices_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Adjustment of Warrants Granted for Services</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_Security12bTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Title of 12(b) Security</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_Security12bTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Title of 12(b) Security</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Percentage of prepayment premium of principal amount outstanding in first year.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Percentage Of Prepayment Premium Of Principal Amount Outstanding In First Year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Percentage of prepayment premium of principal amount outstanding in first year</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandTwentyThreeInducementIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">2023 Inducement Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandTwentyThreeInducementIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Two Thousand Twenty Three Inducement Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TwoThousandTwentyThreeInducementIncentivePlanMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Two Thousand Twenty Three Inducement Incentive Plan [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PreFundedWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Pre funded warrants.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PreFundedWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Pre Funded Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_PreFundedWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Pre Funded Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Fair Value Measurements [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurements [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Measurements [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Stock issued during period value at the market offering expenses.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Value At The Market Offering Expenses</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingExpenses_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">ATM Offering, offering cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrimeRateMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Prime Rate [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PrimeRateMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Wall Street Journal Prime Rate [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeLiabilitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Liability [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeLiabilitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Derivative Liability:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioOneMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Scenario one.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioOneMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario One [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_ScenarioOneMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Available through April 30, 2022 [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AggregatePotentialMilestonePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Aggregate potential milestone payable.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AggregatePotentialMilestonePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Aggregate Potential Milestone Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AggregatePotentialMilestonePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Aggregate potential milestone payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GlobalAndUnitedStatesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Global and united states.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GlobalAndUnitedStatesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Global And United States [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_GlobalAndUnitedStatesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Global and U.S. [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Investing Activities [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">CASH FLOWS FROM INVESTING ACTIVITIES:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CarryingReportedAmountFairValueDisclosureMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Reported Value Measurement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CarryingReportedAmountFairValueDisclosureMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Carrying Value [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember_lbl" xlink:role="http://fasb.org/us-gaap/role/label/axisDefault">Portion at Fair Value Measurement [Member] [Default]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Portion at Fair Value Measurement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_WarrantsToPurchaseCommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Warrants to purchase common stock.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_WarrantsToPurchaseCommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Warrants To Purchase Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_WarrantsToPurchaseCommonStockMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Warrants to Purchase Common Stock [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpenseDebt_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Expense, Debt</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpenseDebt_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest expense related to the loan agreement</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpenseDebt_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Interest Expense, Debt, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Increase decrease in contractual obligation.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Contractual Obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Increase in contractual obligation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantDownRoundFeatureDecreaseInNetIncomeLossToCommonShareholderAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Warrant, Down Round Feature, Decrease in Net Income to Common Shareholder, Amount</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WarrantDownRoundFeatureDecreaseInNetIncomeLossToCommonShareholderAmount_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Deemed dividend due to warrant reset</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NonRefundableCashPaymentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Non-Refundable Cash Payments [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NonRefundableCashPaymentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Non Refundable Cash Payments [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_NonRefundableCashPaymentsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Non refundable cash payments.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Common Stock, Shares, Outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock, shares outstanding</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Common Stock, Shares, Outstanding, Beginning Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Common Stock, Shares, Outstanding, Ending Balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Payment Arrangement [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Stock-Based Compensation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeitures, Weighted Average Grant Date Fair Value</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted Average Grant Date Fair Value, Forfeited</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketEquityOfferingSalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">At the market equity offering sales agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketEquityOfferingSalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">At The Market Equity Offering Sales Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AtTheMarketEquityOfferingSalesAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">At The Market Equity Offering Sales Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementLocationDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Income Statement Location [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncomeStatementLocationDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Income Statement Location</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentType_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentType_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Document Type</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Prepaid Expense, Current</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityInterestRateDescription_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line of Credit Facility, Interest Rate Description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_LineOfCreditFacilityInterestRateDescription_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Interest rate description</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net cash provided by (used in) investing activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Investing Activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaboration and license agreement allocated consideration received to derivative liability.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaboration And License Agreement Allocated Consideration Received To Derivative Liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Allocated consideration received to derivative liability</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentQuarterlyReport_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Document Quarterly Report</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_DocumentQuarterlyReport_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Document Quarterly Report</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Counterparty Name [Domain]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Counterparty Name</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income (Loss) Available to Common Stockholders, Basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net loss attributable to common stockholders - basic</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Net loss attributable to common stockholders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Forfeited in Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Share-based compensation, stock options net of cancellations</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Number of Shares, Forfeited</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Cash Provided by (Used in) Financing Activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net cash provided by financing activities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFilerCategory_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Filer Category</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityFilerCategory_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Entity Filer Category</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Fair value measurements.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurements [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Measurements [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockholdersEquityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Stockholders Equity [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfStockholdersEquityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Summary of Changes in Stockholders' Equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EasternCooperativeOncologyGroupMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Eastern Cooperative Oncology Group.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EasternCooperativeOncologyGroupMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Eastern Cooperative Oncology Group [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_EasternCooperativeOncologyGroupMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Eastern Cooperative Oncology Group [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Variable Rate [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_VariableRateAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Variable Rate</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Short-Term Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Short-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Short-Term Investments, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByAssetClassAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Asset Class [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByAssetClassAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Asset Class</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total other income, net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Nonoperating Income (Expense)</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Liabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_Liabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Payments to Acquire Short-Term Investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Purchases of short and long-term investments</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Equity, Attributable to Parent</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel">Ending balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel">Beginning balance</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total stockholders&#8217; equity</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaboration and license agreement.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaboration And License Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborationAndLicenseAgreementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Incyte Agreement [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NetIncomeLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Net loss</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Nonoperating Income (Expense) [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other income (expense), net:</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Interest expense</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Interest Expense, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BusinessDescriptionAndBasisOfPresentationTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Business Description and Basis of Presentation [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_BusinessDescriptionAndBasisOfPresentationTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Basis of Presentation</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Research and Development Arrangement, Contract to Perform for Others [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfResearchAndDevelopmentArrangementContractToPerformForOthersTable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule Of Research And Development Arrangement Contract To Perform For Others [Table]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CommercializationMilestonesAndTieredRoyalties_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Commercialization milestones and tiered royalties.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CommercializationMilestonesAndTieredRoyalties_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Commercialization Milestones And Tiered Royalties</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CommercializationMilestonesAndTieredRoyalties_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Commercialization milestones and tiered royalties</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdjustmentToAdditionalPaidInCapitalEmployeeWithholdingsEmployeeStockPurchasePlan_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Adjustment to additional paid in capital employee withholdings employee stock purchase plan.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdjustmentToAdditionalPaidInCapitalEmployeeWithholdingsEmployeeStockPurchasePlan_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Adjustment To Additional Paid In Capital Employee Withholdings Employee Stock Purchase Plan</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AdjustmentToAdditionalPaidInCapitalEmployeeWithholdingsEmployeeStockPurchasePlan_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Employee withholdings ESPP</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CorporateDebtSecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Corporate Debt Securities [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CorporateDebtSecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Corporate Bonds [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementOfFinancialPositionAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement of Financial Position [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborativeResearchAndLicenseAgreementsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Collaborative research and license agreements.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborativeResearchAndLicenseAgreementsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Research And License Agreements [Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_CollaborativeResearchAndLicenseAgreementsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaborative Research and License Agreements</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number of Shares Outstanding, Diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Weighted-average number of common shares used to compute net loss per share attributable to common stockholders&#8212;diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Weighted-average number of common shares used to compute net loss per share attributable to common stockholders&#8212;diluted</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Weighted Average Number of Shares Outstanding, Diluted, Total</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CreditFacilityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Credit Facility [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_CreditFacilityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Credit Facility</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesIssuedPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Shares Issued, Price Per Share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SharesIssuedPricePerShare_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Purchase price per share</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AvailableForSaleSecuritiesFairValueDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Available For Sale Securities Fair Value Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AvailableForSaleSecuritiesFairValueDisclosureAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Available For Sale Securities Fair Value Disclosure [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAccountsPayableAndAccruedLiabilitiesTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Accounts Payable and Accrued Liabilities [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfAccountsPayableAndAccruedLiabilitiesTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Schedule of Accrued Expenses and Other Current Liabilities</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Debt Securities, Available-for-Sale, Amortized Cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Amortized Cost</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UseOfEstimates_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Use of Estimates, Policy [Policy Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_UseOfEstimates_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Use of Estimates</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicAndDilutedTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule of Earnings Per Share, Basic and Diluted [Table Text Block]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEarningsPerShareBasicAndDilutedTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Computation of Basic and Diluted Net Loss per Share Attributable to Common Stockholders</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SeriesWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Series Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SeriesWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Series Warrants [Member]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_SeriesWarrantsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Series warrants.</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueDisclosuresAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Disclosures [Abstract]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Hierarchy and NAV [Axis]</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Fair Value Hierarchy and NAV</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivablePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Collaboration receivable (payable), net</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivablePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Collaboration Receivable/Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IncreaseDecreaseInCollaborationReceivablePayable_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Increase Decrease In Collaboration Receivable/Payable</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share-Based Compensation Arrangement by Share-Based Payment Award, Equity Instruments Other than Options, Vested in Period</link:label>
          <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Number of Shares, Vested</link:label>
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          <link:loc xlink:type="locator" xlink:href="https://xbrl.fasb.org/us-gaap/2023/elts/us-gaap-2023.xsd#us-gaap_OtherAccruedLiabilitiesCurrent" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent"/>
        </link:calculationLink>
      </link:linkbase>
    </xsd:appinfo>
  </xsd:annotation>
  <xsd:element id="sndx_TwoThousandAndNineteenPerformanceAwardsMember" name="TwoThousandAndNineteenPerformanceAwardsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_FairValueMeasurementsTable" name="FairValueMeasurementsTable" type="xbrli:stringItemType" substitutionGroup="xbrldt:hypercubeItem" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingThereafter" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_ScenarioThreeMember" name="ScenarioThreeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_SignificantRisksAndUncertaintiesPolicyTextBlock" name="SignificantRisksAndUncertaintiesPolicyTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_DebtInstrumentTrancheTwoMember" name="DebtInstrumentTrancheTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_NumberOfPreFundedWarrantsExercisedForCommonStock" name="NumberOfPreFundedWarrantsExercisedForCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="sndx_CollaborationReceivable" name="CollaborationReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_DebtInstrumentTrancheThreeMember" name="DebtInstrumentTrancheThreeMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_FederalBondsMember" name="FederalBondsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone" name="PotentialMilestonePaymentsUponAchievementOfMilestone" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_PrepaidSubscriptions" name="PrepaidSubscriptions" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_WarrantsToPurchaseCommonStockMember" name="WarrantsToPurchaseCommonStockMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_UcbLicenseAgreementMember" name="UcbLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_PaymentForEndOfTermFees" name="PaymentForEndOfTermFees" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_GrossProceedsFromOffering" name="GrossProceedsFromOffering" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_CowenAndCompanyLLCMember" name="CowenAndCompanyLLCMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingExpenses" name="StockIssuedDuringPeriodValueAtTheMarketOfferingExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_AggregatePotentialMilestonePayable" name="AggregatePotentialMilestonePayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_TwoThousandAndTwentyTwoPerformanceAwardsMember" name="TwoThousandAndTwentyTwoPerformanceAwardsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_CommercializationMilestonesAndTieredRoyalties" name="CommercializationMilestonesAndTieredRoyalties" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_TwoThousandAndSevenStockIncentivePlanMember" name="TwoThousandAndSevenStockIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_NonRefundableCashPaymentsMember" name="NonRefundableCashPaymentsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_AccruedContractualObligation" name="AccruedContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_CurrentPortionOfCapitalLease" name="CurrentPortionOfCapitalLease" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_AccruedClinicalCostsCurrent" name="AccruedClinicalCostsCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_HerculesCapitalIncMember" name="HerculesCapitalIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_NumberOfPreFundedWarrantsCashlessExchangedForCommonStock" name="NumberOfPreFundedWarrantsCashlessExchangedForCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInSecondYear" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false"/>
  <xsd:element id="sndx_CollaborationAndLicenseAgreementMember" name="CollaborationAndLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_UpfrontLicenseFeesPaid" name="UpfrontLicenseFeesPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_PaymentForFullyRefundableFee" name="PaymentForFullyRefundableFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_NumberOfPreFundedWarrantsExchangedForCommonStock" name="NumberOfPreFundedWarrantsExchangedForCommonStock" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_PrepaymentFee" name="PrepaymentFee" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_ClassOfWarrantOrRightIssued" name="ClassOfWarrantOrRightIssued" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear" name="PercentageOfPrepaymentPremiumOfPrincipalAmountOutstandingInFirstYear" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_RefundableFeePayble" name="RefundableFeePayble" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_DepreciationAndRelatedAdjustment" name="DepreciationAndRelatedAdjustment" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_TwoThousandTwentyThreeInducementIncentivePlanMember" name="TwoThousandTwentyThreeInducementIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_IncreaseDecreaseInCollaborationReceivablePayable" name="IncreaseDecreaseInCollaborationReceivablePayable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_MilestoneExpensesPaid" name="MilestoneExpensesPaid" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_FairValueMeasurementsLineItems" name="FairValueMeasurementsLineItems" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_RemainingContractualObligation" name="RemainingContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_EasternCooperativeOncologyGroupMember" name="EasternCooperativeOncologyGroupMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_GlobalAndUnitedStatesMember" name="GlobalAndUnitedStatesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_IncyteCollaborationAgreementAndSharePurchaseAgreementMember" name="IncyteCollaborationAgreementAndSharePurchaseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_OtherReceivablesTextBlock" name="OtherReceivablesTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_LiabilityRelatedToLetterAgreement" name="LiabilityRelatedToLetterAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_AccruedMilestoneCosts" name="AccruedMilestoneCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_FirstAmendmentToLoanAndSecurityAgreementMember" name="FirstAmendmentToLoanAndSecurityAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_LineOfCreditFacilityMinimumCashCovenant" name="LineOfCreditFacilityMinimumCashCovenant" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_LineOfCreditFacilityMinimumCashCovenantWaived" name="LineOfCreditFacilityMinimumCashCovenantWaived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_AllerganLicenseAgreementMember" name="AllerganLicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_ClinicalTrialMember" name="ClinicalTrialMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" name="TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_IncreaseDecreaseInCollaborationReceivable" name="IncreaseDecreaseInCollaborationReceivable" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_RevenueRecognizedUponTransferOfLicenseAgreement" name="RevenueRecognizedUponTransferOfLicenseAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_PrepaidStateAndLocalTaxes" name="PrepaidStateAndLocalTaxes" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_AvailableForSaleSecuritiesFairValueDisclosureAbstract" name="AvailableForSaleSecuritiesFairValueDisclosureAbstract" type="xbrli:stringItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_AdditionalDefaultInterestRate" name="AdditionalDefaultInterestRate" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_AtTheMarketEquityOfferingSalesAgreementMember" name="AtTheMarketEquityOfferingSalesAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_UpfrontPaymentsReceived" name="UpfrontPaymentsReceived" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_ReimbursableCosts" name="ReimbursableCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_AdjustmentToAdditionalPaidInCapitalEmployeeWithholdingsEmployeeStockPurchasePlan" name="AdjustmentToAdditionalPaidInCapitalEmployeeWithholdingsEmployeeStockPurchasePlan" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_PercentageOfEndOfTermChargeToFutureDraws" name="PercentageOfEndOfTermChargeToFutureDraws" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_SignificantAgreementsDisclosureTextBlock" name="SignificantAgreementsDisclosureTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_DevelopmentAndRegulaotryMilestonesExpenses" name="DevelopmentAndRegulaotryMilestonesExpenses" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_AtTheMarketStockOfferingMember" name="AtTheMarketStockOfferingMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_SeriesWarrantsMember" name="SeriesWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_NoncashOperatingLeaseExpense" name="NoncashOperatingLeaseExpense" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_BayerPharmaAgMember" name="BayerPharmaAgMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_LoanAndSecurityAgreementMember" name="LoanAndSecurityAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_CollaborationPayableCurrent" name="CollaborationPayableCurrent" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_ReimbursableExpenseDueFromCounterparty" name="ReimbursableExpenseDueFromCounterparty" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_CollaborationExpenseDueToCounterparty" name="CollaborationExpenseDueToCounterparty" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_ContractualObligationsTerm" name="ContractualObligationsTerm" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_LicenseAgreementMember" name="LicenseAgreementMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_CapitalLeaseLessCurrentPortion" name="CapitalLeaseLessCurrentPortion" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_CollaborativeResearchAndLicenseAgreementsTextBlock" name="CollaborativeResearchAndLicenseAgreementsTextBlock" type="dtr-types:textBlockItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_MilestonePaymentDue" name="MilestonePaymentDue" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_StockIssuedDuringPeriodSharesAtTheMarketOfferingNet" name="StockIssuedDuringPeriodSharesAtTheMarketOfferingNet" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_UpfrontPaymentsToBeReturnedUponTerminationOfAgreement" name="UpfrontPaymentsToBeReturnedUponTerminationOfAgreement" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_UpfrontPayments" name="UpfrontPayments" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_DevelopmentCostsAssociatedWithClinicalTrials" name="DevelopmentCostsAssociatedWithClinicalTrials" type="dtr-types:percentItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability" name="CollaborationAndLicenseAgreementAllocatedConsiderationReceivedToDerivativeLiability" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_ExerciseOfPrefundedWarrantsShares" name="ExerciseOfPrefundedWarrantsShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_UCBBiopharmaMember" name="UCBBiopharmaMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_LicenseAgreementsExpirationPeriod" name="LicenseAgreementsExpirationPeriod" type="xbrli:durationItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_StockIssuedDuringPeriodValueAtTheMarketOfferingNet" name="StockIssuedDuringPeriodValueAtTheMarketOfferingNet" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_VestingOfRestrictedStockUnitsShares" name="VestingOfRestrictedStockUnitsShares" type="xbrli:sharesItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_PrepaidConferenceMeetingCosts" name="PrepaidConferenceMeetingCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_IncyteCorporationMember" name="IncyteCorporationMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_VitaePharmaceuticalsIncMember" name="VitaePharmaceuticalsIncMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_OfferingPricePerShare" name="OfferingPricePerShare" type="dtr-types:perShareItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false"/>
  <xsd:element id="sndx_EmployeeStockPurchasePlanMember" name="EmployeeStockPurchasePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_ScenarioOneMember" name="ScenarioOneMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_ScenarioTwoMember" name="ScenarioTwoMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_PreFundedWarrantsMember" name="PreFundedWarrantsMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_IncreaseDecreaseInContractualObligation" name="IncreaseDecreaseInContractualObligation" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_TwoThousandFifteenOmnibusIncentivePlanMember" name="TwoThousandFifteenOmnibusIncentivePlanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_AccruedResearchAndDevelopmentCosts" name="AccruedResearchAndDevelopmentCosts" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="instant" nillable="true" abstract="false" xbrli:balance="credit"/>
  <xsd:element id="sndx_EmployeesMember" name="EmployeesMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
  <xsd:element id="sndx_EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet" name="EmployeeServiceShareBasedCompensationRecognizedStockOptionsNotMet" type="xbrli:monetaryItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="false" xbrli:balance="debit"/>
  <xsd:element id="sndx_TermLoanMember" name="TermLoanMember" type="dtr-types:domainItemType" substitutionGroup="xbrli:item" xbrli:periodType="duration" nillable="true" abstract="true"/>
</xsd:schema>
