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    <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="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="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_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="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_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="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_NatureOfOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Nature Of Operations</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="sndx_InsiderTradingArrAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Insider Trading Arr [Abstract]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_InsiderTradingArrAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Insider Trading Arr [Abstract]</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_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="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Related Liabilities Current</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accrued compensation and related costs</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_EmployeeRelatedLiabilitiesCurrent_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Employee-related Liabilities, Current, Total</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrSecuritiesAggAvailAmt_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trd Arr Securities Agg Avail Amt</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrSecuritiesAggAvailAmt_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trd Arr Securities Agg Avail Amt</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrSecuritiesAggAvailAmt_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trd Arr Securities Agg Avail Amt</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/label">Employee Stock Option [Member]</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/terseLabel">Options to Purchase Common Stock [Member]</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_LiabilitiesAndStockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities And Stockholders 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="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="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="us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number Of Shares Outstanding [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="dei_EntityIncorporationStateCountryCode_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Incorporation State 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_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="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_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_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="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="us-gaap_DerivativeLiabilities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Liabilities</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="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_SaleOfStockNameOfTransactionDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Sale Of Stock Name Of Transaction [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="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="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="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="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_EarningsPerShareBasicLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Earnings Per Share Basic [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="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_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_IncreaseDecreaseInContractWithCustomerLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Contract With Customer Liability</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_IncreaseDecreaseInContractWithCustomerLiability_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Deferred revenue</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="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_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="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>
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    <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_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="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="us-gaap_TypeOfArrangementAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Type Of Arrangement [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="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_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_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_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="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_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_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="us-gaap_DisclosureOfCompensationRelatedCostsSharebasedPaymentsAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]</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_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="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="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_AvailableForSaleDebtSecuritiesAmortizedCostBasis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Available For Sale Debt Securities Amortized Cost Basis</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="srt_RangeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Range [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="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="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds From Sale And Maturity Of Available For Sale Securities</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="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="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_ScheduleOfEarningsPerShareBasicByCommonClassTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Earnings Per Share Basic By Common Class [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="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_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="us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Antidilutive Securities Excluded From Computation Of Earnings Per Share [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="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_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement With Unobservable Inputs Reconciliations 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_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="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_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_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 June 30, 2023 and December 31, 2022</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</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">Adjustments To Additional Paid In Capital Sharebased Compensation Requisite Service Period Recognition Value</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_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="srt_RepurchaseAgreementCounterpartyNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Repurchase Agreement 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="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="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="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="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="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_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="dei_EntityFileNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity 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="srt_ScenarioUnspecifiedDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario Unspecified [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_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="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="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="srt_SegmentGeographicalDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Segment 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="srt_RangeMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Range [Member]</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="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_SubsidiarySaleOfStockAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsidiary 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="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="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_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period</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_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_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_MtrlTermsOfTrdArr1TextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Mtrl Terms of Trd Arr 1 [Text Block]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MtrlTermsOfTrdArr1TextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Mtrl Terms of Trd Arr 1 [Text Block]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_MtrlTermsOfTrdArr1TextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Mtrl Terms of Trd Arr 1 [Text Block]</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 [Text Block]</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">Compensation Expense by Type of Award</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="us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Antidilutive Securities Excluded From Computation Of Earnings Per Share By 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_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_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 [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_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_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Available For Sale Debt Securities 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="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_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_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Collaborative Arrangements And Noncollaborative Arrangement Transactions [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="us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Weighted Average Number Of Diluted Shares Outstanding</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_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_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="us-gaap_GeneralAndAdministrativeExpenseMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">General And Administrative Expense [Member]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_GeneralAndAdministrativeExpenseMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">General and Administrative [Member]</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_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>
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    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrTerminationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trd Arr Termination Date</link:label>
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    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrTerminationDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trd Arr Termination 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/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_CashCashEquivalentsRestrictedCashAndRestrictedCashEquivalentsIncludingDisposalGroupAndDiscontinuedOperations_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Cash Cash Equivalents Restricted Cash And Restricted Cash Equivalents Including Disposal Group And Discontinued Operations</link:label>
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    <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>
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    <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="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="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="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_StockIssuedDuringPeriodSharesNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stock Issued During Period Shares New Issues</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Common stock shares purchased</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xlink:role="http://www.xbrl.org/2003/role/verboseLabel">Common stock, new shares issued</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">Carrying Reported Amount Fair Value Disclosure [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="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_FairValueByFairValueHierarchyLevelAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value By Fair Value Hierarchy Level [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="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="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_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_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="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_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_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="srt_StatementScenarioAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement 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_ScheduleOfCollaborativeArrangementsAndNoncollaborativeArrangementTransactionsTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Collaborative Arrangements And Noncollaborative Arrangement Transactions [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="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_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_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_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="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_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="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_FairValueAssetsMeasuredOnRecurringBasisUnobservableInputReconciliationByAssetClassDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Assets Measured On Recurring Basis Unobservable Input Reconciliation By 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="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_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_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="sndx_AllTradingArrangementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">All Trading Arrangement [Member]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AllTradingArrangementMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">All Trading Arrangement [Member]</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">Stockholders Equity Note Disclosure [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="us-gaap_StockholdersEquity_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity</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>
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    <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="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">Accumulated Other Comprehensive Income [Member]</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">Fair Value By 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="dei_EntityTaxIdentificationNumber_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Tax Identification Number</link:label>
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    <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_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>
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    <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>
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    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement Equity Components [Axis]</link:label>
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    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_DerivativeGainLossOnDerivativeNet_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Derivative, Gain (Loss) on Derivative, Net, Total</link:label>
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    <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="sndx_ExerciseOfPrefundedWarrantsShares_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Exercise Of Prefunded Warrants, Shares</link:label>
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    <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 [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="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="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="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="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_ProceedsFromSaleOfMachineryAndEquipment_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Proceeds from sales of equipment</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ProceedsFromSaleOfMachineryAndEquipment_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Proceeds from Sale of Machinery and Equipment</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="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="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_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="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_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_InsiderTradingArrLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Insider Trading Arr [Line Items]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_InsiderTradingArrLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Insider Trading Arr [Line Items]</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="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_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="us-gaap_NonoperatingIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Total other income (expense), 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_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="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_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="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_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="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="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="sndx_TradingArrByInd1Table_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trading Arr By Ind1 [Table]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TradingArrByInd1Table_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trading Arr By Ind1 [Table]</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/label">Net Income Loss</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/terseLabel">Net loss</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="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="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_FairValueMeasurementInputsDisclosureTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurement Inputs Disclosure [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_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_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="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_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_ArrangementsAndNonarrangementTransactionsMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Arrangements And Nonarrangement Transactions [Member]</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="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_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="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="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="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_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="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_TrdArrIndTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trd Arr Ind Title</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrIndTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trd Arr Ind Title</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrIndTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trd Arr Ind Title</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 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/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="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="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="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Compensation Arrangements By Share Based Payment Award Award Type And Plan Name [Domain]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Award Type</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="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="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="dei_Security12bTitle_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Security12b Title</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="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="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="us-gaap_DerivativeLiabilitiesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Derivative Liabilities [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="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_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_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_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_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="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_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="srt_StatementGeographicalAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Statement 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_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="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_InterestPaidNet_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Interest Paid Net</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_InterestPaidNet_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Cash paid for interest</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="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="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_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_AllIndividualMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">All Individual [Member]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_AllIndividualMember_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">All Individual [Member]</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">Allocated Share Based Compensation 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="us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Accumulated other comprehensive (loss)</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">Accumulated Other Comprehensive Income Loss Available For Sale Securities Adjustment Net Of Tax</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="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xlink:role="http://www.xbrl.org/2003/role/label">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_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="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_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_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="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_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="us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized Period For Recognition1</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="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Comprehensive Income Loss Net Of Tax Period Increase Decrease [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="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="sndx_TrdArrIndName_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trd Arr Ind Name</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrIndName_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trd Arr Ind Name</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrIndName_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trd Arr Ind Name</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 R S U [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_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="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_ScheduleOfShareBasedCompensationRestrictedStockUnitsAwardActivityTableTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Share Based Compensation Restricted Stock Units Award 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_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="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_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_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_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_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_FairValueInputsLevel2Member_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Inputs Level2 [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_AvailableForSaleSecuritiesDebtSecurities_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Available For Sale Securities Debt Securities</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_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="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="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="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_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_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_USTreasurySecuritiesMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">U S 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_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="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="sndx_IndividualsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Individuals [Axis]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_IndividualsAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Individuals [Axis]</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_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_StockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Stockholders Equity [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="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="dei_EntityAddressAddressLine2_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address Address Line2</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="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="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="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_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="us-gaap_AwardTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Award Type [Axis]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_AwardTypeAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Award Type</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNoncashIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2009/role/negatedLabel">Other</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNoncashIncomeExpense_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_OtherNoncashIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Other Noncash Income (Expense)</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_OtherNoncashIncomeExpense_lbl" xlink:role="http://www.xbrl.org/2003/role/totalLabel">Other Noncash Income (Expense), Total</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Subsequent Event [Member]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_SubsequentEventMember_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Subsequent Event [Member]</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">Employee Service Share Based Compensation Nonvested Awards Total Compensation Cost Not Yet Recognized</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="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_TradingArrAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trading Arr [Axis]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TradingArrAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trading Arr [Axis]</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 income (expense), net</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_NonoperatingIncomeExpenseAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Other (expense) income, net:</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_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_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="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_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_NetIncomeLossAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Net Income Loss [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="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="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="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_ScheduleOfAvailableForSaleSecuritiesLineItems_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Available For Sale Securities [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="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_DisclosureOfCompensationRelatedCostsShareBasedPaymentsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Disclosure Of Compensation Related Costs Share Based Payments [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_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="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="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/terseLabel">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/totalLabel">General and Administrative Expense, Total</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="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="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="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="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="srt_ScenarioForecastMember_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Scenario 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="sndx_TrdArrAdoptionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Trd Arr Adoption Date</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrAdoptionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Trd Arr Adoption Date</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_TrdArrAdoptionDate_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Trd Arr Adoption Date</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_Rule10B51ArrTrmntdFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Rule 10b5-1 Arr Trmntd [Flag]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_Rule10B51ArrTrmntdFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Rule 10b5-1 Arr Trmntd [Flag]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_Rule10B51ArrTrmntdFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Rule 10b5-1 Arr Trmntd [Flag]</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_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="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="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="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_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="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="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="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_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="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="sndx_Rule10B51ArrAdoptedFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Rule 10b5-1 Arr Adopted [Flag]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_Rule10B51ArrAdoptedFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Rule 10b5-1 Arr Adopted [Flag]</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="sndx_Rule10B51ArrAdoptedFlag_lbl" xlink:role="http://www.xbrl.org/2003/role/documentation">Rule 10b5-1 Arr Adopted [Flag]</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_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_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_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="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_LineOfCreditFacilityAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Line Of Credit Facility [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_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_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [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_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="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_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_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_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Increase Decrease In Prepaid Deferred 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_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="dei_EntityIncorporationDateOfIncorporation_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Incorporation Date Of Incorporation</link:label>
    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="dei_EntityIncorporationDateOfIncorporation_lbl" xlink:role="http://www.xbrl.org/2003/role/terseLabel">Date of incorporation</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="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="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_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="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_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="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="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_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="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_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_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_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_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_LiabilitiesAndStockholdersEquityAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Liabilities And Stockholders 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="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="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="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_ProfitLoss_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Profit 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/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="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="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_FairValueMeasurementsFairValueHierarchyDomain_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value Measurements Fair Value Hierarchy [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_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="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_ShareBasedArrangementsToObtainGoodsAndServicesAbstract_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Share Based Arrangements To Obtain Goods And Services [Abstract]</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_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="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="us-gaap_FairValueByMeasurementBasisAxis_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Fair Value By 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_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>
    <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="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_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="dei_EntityAddressAddressLine3_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Entity Address Address Line3</link:label>
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    <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>
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    <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>
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    <link:label xml:lang="en-US" xlink:type="resource" xlink:label="us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTextBlock_lbl" xlink:role="http://www.xbrl.org/2003/role/label">Schedule Of Employee Service Share Based Compensation Allocation Of Recognized Period Costs [Text Block]</link:label>
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    <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>
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    <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>
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    <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 With Relationship To Entity [Domain]</link:label>
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    <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>
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