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  <element name="IncomeTaxesTable" id="sndx_IncomeTaxesTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <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="IndemnificationObligationsClaimsReserves" id="sndx_IndemnificationObligationsClaimsReserves" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="KhkagreementMember" id="sndx_KhkagreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" 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="MonthlyRentAbatement" id="sndx_MonthlyRentAbatement" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" 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="OperatingLeasesCommencementDate" id="sndx_OperatingLeasesCommencementDate" type="xbrli:dateItemType" 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="OvernightInvestmentsMoneyMarketFundsAndCorporateBondsMember" id="sndx_OvernightInvestmentsMoneyMarketFundsAndCorporateBondsMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="PercentageOfIncomeTaxContingencyRealized" id="sndx_PercentageOfIncomeTaxContingencyRealized" 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="ProceedsFromLicenseAgreement" id="sndx_ProceedsFromLicenseAgreement" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="QuarterlyFinancialInformationLineItems" id="sndx_QuarterlyFinancialInformationLineItems" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="QuarterlyFinancialInformationTable" id="sndx_QuarterlyFinancialInformationTable" type="xbrli:stringItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrldt:hypercubeItem"/>
  <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="RentAbatements" id="sndx_RentAbatements" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="instant" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="RevaluationDateOfWarrant" id="sndx_RevaluationDateOfWarrant" type="xbrli:dateItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ReverseStockSplitPolicyTextBlock" id="sndx_ReverseStockSplitPolicyTextBlock" type="nonnum:textBlockItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ScheduleOfFutureMinimumLeasePaymentsForCapitalAndOperatingLeasesTableTextBlock" id="sndx_ScheduleOfFutureMinimumLeasePaymentsForCapitalAndOperatingLeasesTableTextBlock" type="nonnum:textBlockItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeniorSecuredTermLoanFacilityMember" id="sndx_SeniorSecuredTermLoanFacilityMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesAConvertiblePreferredStockMember" id="sndx_SeriesAConvertiblePreferredStockMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesAOnePreferredStockMember" id="sndx_SeriesAOnePreferredStockMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesB1PurchaseAgreementMember" id="sndx_SeriesB1PurchaseAgreementMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesBOnePreferredStockMember" id="sndx_SeriesBOnePreferredStockMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesCOneConvertiblePreferredStockAugustTwoThousandFifteenMember" id="sndx_SeriesCOneConvertiblePreferredStockAugustTwoThousandFifteenMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesCOneConvertiblePreferredStockJuneTwoThousandFifteenMember" id="sndx_SeriesCOneConvertiblePreferredStockJuneTwoThousandFifteenMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="SeriesCOnePreferredStockMember" id="sndx_SeriesCOnePreferredStockMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncreasePercentageInNumberOfSharesAvailableForGrant" id="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardAnnualIncreasePercentageInNumberOfSharesAvailableForGrant" type="num:percentItemType" 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="SolarCapitalLtdMember" id="sndx_SolarCapitalLtdMember" type="nonnum:domainItemType" abstract="true" 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="StockholdersMember" id="sndx_StockholdersMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" 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="TemporaryEquityConversionOfNotesIntoPreferredStockShares" id="sndx_TemporaryEquityConversionOfNotesIntoPreferredStockShares" type="xbrli:sharesItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityConversionOfNotesIntoPreferredStockValue" id="sndx_TemporaryEquityConversionOfNotesIntoPreferredStockValue" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityIssuanceCost" id="sndx_TemporaryEquityIssuanceCost" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityIssuanceDate1" id="sndx_TemporaryEquityIssuanceDate1" type="invest:dateStringItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityIssuanceDates" id="sndx_TemporaryEquityIssuanceDates" type="xbrli:gYearMonthItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityIssuancePricePerShare" id="sndx_TemporaryEquityIssuancePricePerShare" type="num:perShareItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" id="sndx_TemporaryEquityStockIssuedDuringPeriodSharesNewIssues" type="xbrli:sharesItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TermALoanMember" id="sndx_TermALoanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TermcLoanMember" id="sndx_TermcLoanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TermLoanMember" id="sndx_TermLoanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TermLoanProceedsAllocatedToDerivativeLiability" id="sndx_TermLoanProceedsAllocatedToDerivativeLiability" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="credit" 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="TwoThousandFifteenOmnibusIncentivePlanMember" id="sndx_TwoThousandFifteenOmnibusIncentivePlanMember" type="nonnum:domainItemType" abstract="true" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="TwoThousandFourteenNotesMember" id="sndx_TwoThousandFourteenNotesMember" 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="WarrantExpirationTerm" id="sndx_WarrantExpirationTerm" type="xbrli:durationItemType" abstract="false" xbrli:periodType="duration" nillable="true" substitutionGroup="xbrli:item"/>
  <element name="WriteOffOfDeferredCostsAssociatedWithPostponedIPO" id="sndx_WriteOffOfDeferredCostsAssociatedWithPostponedIPO" type="xbrli:monetaryItemType" abstract="false" xbrli:periodType="duration" xbrli:balance="debit" nillable="true" substitutionGroup="xbrli:item"/>
</schema>
