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  <import namespace="http://fasb.org/us-types/2017-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2017/elts/us-types-2017-01-31.xsd"/>
  <import namespace="http://www.xbrl.org/dtr/type/non-numeric" schemaLocation="http://www.xbrl.org/dtr/type/nonNumeric-2009-12-16.xsd"/>
  <import namespace="http://www.xbrl.org/dtr/type/numeric" schemaLocation="http://www.xbrl.org/dtr/type/numeric-2009-12-16.xsd"/>
  <import namespace="http://xbrl.org/2005/xbrldt" schemaLocation="http://www.xbrl.org/2005/xbrldt-2005.xsd"/>
  <import namespace="http://fasb.org/us-gaap/2017-01-31" schemaLocation="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/country/2017-01-31" schemaLocation="http://xbrl.sec.gov/country/2017/country-2017-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/currency/2017-01-31" schemaLocation="http://xbrl.sec.gov/currency/2017/currency-2017-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/dei/2014-01-31" schemaLocation="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/exch/2017-01-31" schemaLocation="http://xbrl.sec.gov/exch/2017/exch-2017-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/invest/2013-01-31" schemaLocation="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/naics/2017-01-31" schemaLocation="http://xbrl.sec.gov/naics/2017/naics-2017-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/sic/2011-01-31" schemaLocation="http://xbrl.sec.gov/sic/2011/sic-2011-01-31.xsd"/>
  <import namespace="http://xbrl.sec.gov/stpr/2011-01-31" schemaLocation="http://xbrl.sec.gov/stpr/2011/stpr-2011-01-31.xsd"/>
  <element name="AccruedClinicalCostsCurrent" id="sndx_AccruedClinicalCostsCurrent" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="AggregatePotentialMilestonePayable" id="sndx_AggregatePotentialMilestonePayable" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="AllerganLicenseAgreementMember" id="sndx_AllerganLicenseAgreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="AnticipatedPeriodOfApproval" id="sndx_AnticipatedPeriodOfApproval" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="AtTheMarketEquityOfferingSalesAgreementMember" id="sndx_AtTheMarketEquityOfferingSalesAgreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="AvailableForSaleSecuritiesFairValueDisclosureAbstract" id="sndx_AvailableForSaleSecuritiesFairValueDisclosureAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="BayerPharmaAgMember" id="sndx_BayerPharmaAgMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ClinicalTrialMember" id="sndx_ClinicalTrialMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="CommonStockAvailableForSaleUnderSalesAgreement" id="sndx_CommonStockAvailableForSaleUnderSalesAgreement" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="CommonStockWarrantLiabilityMember" id="sndx_CommonStockWarrantLiabilityMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ContractualObligationsTerm" id="sndx_ContractualObligationsTerm" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="CorporateBondsAndCommercialPaperMember" id="sndx_CorporateBondsAndCommercialPaperMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="CostsRelatedToCostSharingCollaborations" id="sndx_CostsRelatedToCostSharingCollaborations" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="CowenAndCompanyLLCMember" id="sndx_CowenAndCompanyLLCMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="DocumentAndEntityInformationAbstract" id="sndx_DocumentAndEntityInformationAbstract" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="EasternCooperativeOncologyGroupMember" id="sndx_EasternCooperativeOncologyGroupMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="EmployeeBenefitPlansLineItems" id="sndx_EmployeeBenefitPlansLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="EmployeeStockPurchasePlanMember" id="sndx_EmployeeStockPurchasePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="FairValueAdjustmentOfWarrantsAndDerivativeLiabilities" id="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="FairValueMeasurementsLineItems" id="sndx_FairValueMeasurementsLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="FairValueMeasurementsTable" id="sndx_FairValueMeasurementsTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <element name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" id="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityReclassificationToEquity" id="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityReclassificationToEquity" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="FollowOnPublicOfferingMember" id="sndx_FollowOnPublicOfferingMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="GrossProceedsFromIssuanceOfCommonStock" id="sndx_GrossProceedsFromIssuanceOfCommonStock" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="IncreaseDecreaseInContractualObligation" id="sndx_IncreaseDecreaseInContractualObligation" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="IncreaseDecreaseInResearchAndDevelopmentExpense" id="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses" id="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="KyowaHakkoKirinCoLtdMember" id="sndx_KyowaHakkoKirinCoLtdMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="LicenseAgreementMember" id="sndx_LicenseAgreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="LicenseAgreementsExpirationPeriod" id="sndx_LicenseAgreementsExpirationPeriod" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="MilestonePaymentDue" id="sndx_MilestonePaymentDue" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="NatureOfBusinessLineItems" id="sndx_NatureOfBusinessLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="NatureOfBusinessTable" id="sndx_NatureOfBusinessTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <element name="NonRefundableUpFrontPaymentReceived" id="sndx_NonRefundableUpFrontPaymentReceived" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="NumberOfExecutivesOptionsVested" id="sndx_NumberOfExecutivesOptionsVested" type="xbrli:integerItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="NumberOfRevenueArrangement" id="sndx_NumberOfRevenueArrangement" type="xbrli:integerItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="OtherAssetsPolicyTextBlock" id="sndx_OtherAssetsPolicyTextBlock" type="nonnum:textBlockItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="OvernightInvestmentsAndMoneyMarketFundsMember" id="sndx_OvernightInvestmentsAndMoneyMarketFundsMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares" id="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares" type="num:percentItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PercentageOfSharesIssuableOnConversionOfWarrant" id="sndx_PercentageOfSharesIssuableOnConversionOfWarrant" type="num:percentItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" id="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PotentialMilestonePaymentsReceivable" id="sndx_PotentialMilestonePaymentsReceivable" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PotentialMilestonePaymentsUponAchievementOfMilestone" id="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet" id="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="RecentAccountingStandardsExpectedDateOfAdoption" id="sndx_RecentAccountingStandardsExpectedDateOfAdoption" type="xbrli:dateItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital" id="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ReimbursableCosts" id="sndx_ReimbursableCosts" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="RemainingContractualObligation" id="sndx_RemainingContractualObligation" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ScheduleOfEmployeeBenefitPlansTable" id="sndx_ScheduleOfEmployeeBenefitPlansTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <element name="SeriesBOnePreferredStockMember" id="sndx_SeriesBOnePreferredStockMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SignificantAccountingPoliciesPolicyTextBlock" id="sndx_SignificantAccountingPoliciesPolicyTextBlock" type="nonnum:textBlockItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SignificantAgreementsDisclosureTextBlock" id="sndx_SignificantAgreementsDisclosureTextBlock" type="nonnum:textBlockItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="StockBasedCompensationDueToAcceleratedVestingOfStockOptions" id="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram" id="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram" type="xbrli:sharesItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram" id="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SuccessFeePayableMember" id="sndx_SuccessFeePayableMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SummaryOfSignificantAccountingPoliciesLineItems" id="sndx_SummaryOfSignificantAccountingPoliciesLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SummaryOfSignificantAccountingPoliciesTable" id="sndx_SummaryOfSignificantAccountingPoliciesTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <element name="TermLoanMember" id="sndx_TermLoanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TwoThousandAndSevenStockIncentivePlanMember" id="sndx_TwoThousandAndSevenStockIncentivePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TwoThousandFifteenEmployeeStockPurchasePlanMember" id="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TwoThousandFifteenOmnibusIncentivePlanMember" id="sndx_TwoThousandFifteenOmnibusIncentivePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" id="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UCBBiopharmaMember" id="sndx_UCBBiopharmaMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UCBLicenseAgreementMember" id="sndx_UCBLicenseAgreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UnderwritingDiscountsAndCommissions" id="sndx_UnderwritingDiscountsAndCommissions" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UpfrontLicenseFeesPaid" id="sndx_UpfrontLicenseFeesPaid" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UpfrontPaymentAllocatedToLicenseFee" id="sndx_UpfrontPaymentAllocatedToLicenseFee" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UpfrontPayments" id="sndx_UpfrontPayments" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="UpfrontPaymentsReceived" id="sndx_UpfrontPaymentsReceived" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="VestingOfRestrictedStockValue" id="sndx_VestingOfRestrictedStockValue" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="VitaePharmaceuticalsIncMember" id="sndx_VitaePharmaceuticalsIncMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="WarrantExpirationTerm" id="sndx_WarrantExpirationTerm" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
</schema>
