<?xml version="1.0" encoding="US-ASCII"?>
<!-- DFS Xcelerate Taxonomy Labels Linkbase - http://www.dfsco.com/financial-services/ -->
<!-- Version: 6.23.6 -->
<!-- Round: 3 -->
<!-- Creation date: 2017-11-07T05:27:53Z -->
<!-- Copyright (c) 2017 Donnelley Financial, LLC. All Rights Reserved. -->
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_AccruedClinicalCostsCurrent_lbl" xml:lang="en-US" id="id_11438736_2538618_3_1">Accrued clinical costs current.</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl" xml:lang="en-US" id="id_11438736_2432328_1_1">Accrued Liabilities and Other Liabilities</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl" xml:lang="en-US" id="id_11438736_2432328_6_1">Total prepaid expenses and other current assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_AccruedLiabilitiesAndOtherLiabilities" xlink:to="us-gaap_AccruedLiabilitiesAndOtherLiabilities_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccruedProfessionalFeesCurrent" xlink:label="us-gaap_AccruedProfessionalFeesCurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccruedProfessionalFeesCurrent_lbl" xml:lang="en-US" id="id_11438736_2432762_1_1">Accrued Professional Fees, Current</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2432777_1_1">Accumulated Other Comprehensive Income (Loss), Available-for-sale Securities Adjustment, Net of Tax</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2432777_2_1">Accumulated other comprehensive income</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AccumulatedOtherComprehensiveIncomeMember_lbl" xml:lang="en-US" id="id_11438736_2433011_1_1">AOCI Attributable to Parent [Member]</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xml:lang="en-US" id="id_11438736_2432790_1_1">Additional Paid in Capital, Common Stock</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdditionalPaidInCapitalCommonStock_lbl" xml:lang="en-US" id="id_11438736_2432790_2_1">Additional paid-in capital</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="us-gaap_AdditionalPaidInCapitalMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdditionalPaidInCapitalMember_lbl" xml:lang="en-US" id="id_11438736_2432358_1_1">Additional Paid-in Capital [Member]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_lbl" xml:lang="en-US" id="id_11438736_2432802_2_1">Stock-based compensation expense</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_lbl" xml:lang="en-US" id="id_11438736_2432804_1_1">Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalStockIssuedIssuanceCosts_lbl" xml:lang="en-US" id="id_11438736_2432804_2_1">Net offering costs</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_lbl" xml:lang="en-US" id="id_11438736_2432602_1_1">Adjustments to Additional Paid in Capital, Warrant Issued</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_lbl" xml:lang="en-US" id="id_11438736_2432602_2_1">Reclassification of common stock warrant liability to additional paid-in capital</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" id="id_11438736_2433031_1_1">Adjustments to Reconcile Net Income (Loss) to Cash Provided by (Used in) Operating Activities [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_lbl" xml:lang="en-US" id="id_11438736_2433031_2_1">Adjustments to reconcile net loss to net cash from operating activities:</label>
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    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_AggregatePotentialMilestonePayable" xlink:label="sndx_AggregatePotentialMilestonePayable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_AggregatePotentialMilestonePayable_lbl" xml:lang="en-US" id="id_11438736_2536365_3_1">Aggregate potential milestone payable.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_AggregatePotentialMilestonePayable_lbl" xml:lang="en-US" id="id_11438736_2536365_1_1">Aggregate Potential Milestone Payable</label>
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    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_AllerganLicenseAgreementMember" xlink:label="sndx_AllerganLicenseAgreementMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_AllerganLicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2897353_3_1">Allergan liicense agreement.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_AllerganLicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2897353_1_1">Allergan License Agreement [Member]</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_AllocatedShareBasedCompensationExpense_lbl" xml:lang="en-US" id="id_11438736_2432632_1_1">Allocated Share-based Compensation Expense</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_AmendmentFlag" xlink:label="dei_AmendmentFlag"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_AmendmentFlag_lbl" xml:lang="en-US" id="id_11438736_2432431_1_1">Amendment Flag</label>
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    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_AnticipatedPeriodOfApproval" xlink:label="sndx_AnticipatedPeriodOfApproval"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_AnticipatedPeriodOfApproval_lbl" xml:lang="en-US" id="id_11438736_2867741_3_1">Anticipated period of approval.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_AnticipatedPeriodOfApproval_lbl" xml:lang="en-US" id="id_11438736_2867741_1_1">Anticipated Period of Approval</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_AnticipatedPeriodOfApproval_lbl" xml:lang="en-US" id="id_11438736_2867741_2_1">Anticipated period of approval</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesAuthorized_lbl" xml:lang="en-US" id="id_11438736_2434172_2_1">Common stock, shares authorized</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" id="id_11438736_2434837_1_1">Common Stock, Shares, Issued</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesIssued_lbl" xml:lang="en-US" id="id_11438736_2434837_2_1">Common stock, shares issued</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockSharesIssued" xlink:to="us-gaap_CommonStockSharesIssued_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="us-gaap_CommonStockSharesOutstanding"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2434173_1_1">Common Stock, Shares, Outstanding</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockSharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2434173_2_1">Common stock, shares outstanding</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockSharesOutstanding" xlink:to="us-gaap_CommonStockSharesOutstanding_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CommonStockValue" xlink:label="us-gaap_CommonStockValue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CommonStockValue_lbl" xml:lang="en-US" id="id_11438736_2434840_1_1">Common Stock, Value, Issued</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CommonStockValue_lbl" xml:lang="en-US" id="id_11438736_2434840_2_1">Common stock, $0.0001 par value, 100,000,000 shares authorized; 22,313,001 and 18,215,181 shares outstanding at September 30, 2017 and December 31, 2016, respectively</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CommonStockValue" xlink:to="us-gaap_CommonStockValue_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_CommonStockWarrantLiabilityMember" xlink:label="sndx_CommonStockWarrantLiabilityMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_CommonStockWarrantLiabilityMember_lbl" xml:lang="en-US" id="id_11438736_2653830_3_1">Common Stock Warrant Liability</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_CommonStockWarrantLiabilityMember_lbl" xml:lang="en-US" id="id_11438736_2653830_1_1">Common Stock Warrant Liability [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_CommonStockWarrantLiabilityMember_lbl" xml:lang="en-US" id="id_11438736_2653830_2_1">Common Stock Warrant Liability [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_CommonStockWarrantLiabilityMember" xlink:to="sndx_CommonStockWarrantLiabilityMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock" xlink:label="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock_lbl" xml:lang="en-US" id="id_11438736_2434417_1_1">Compensation and Employee Benefit Plans, Other than Share-based Compensation [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock_lbl" xml:lang="en-US" id="id_11438736_2434417_2_1">Employee Stock Purchase Plan</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock" xlink:to="us-gaap_CompensationAndEmployeeBenefitPlansOtherThanShareBasedCompensationTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CompensationAndRetirementDisclosureAbstract" xlink:label="us-gaap_CompensationAndRetirementDisclosureAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CompensationAndRetirementDisclosureAbstract_lbl" xml:lang="en-US" id="id_11438736_2434418_1_1">Retirement Benefits [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CompensationAndRetirementDisclosureAbstract_lbl" xml:lang="en-US" id="id_11438736_2434418_2_1">Retirement Benefits [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CompensationAndRetirementDisclosureAbstract" xlink:to="us-gaap_CompensationAndRetirementDisclosureAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2434850_1_1">Comprehensive Income (Loss), Net of Tax, Attributable to Parent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_ComprehensiveIncomeNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2434850_6_1">Comprehensive loss</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ComprehensiveIncomeNetOfTax" xlink:to="us-gaap_ComprehensiveIncomeNetOfTax_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConsolidationPolicyTextBlock" xlink:label="us-gaap_ConsolidationPolicyTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ConsolidationPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2434219_1_1">Consolidation, Policy [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ConsolidationPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2434219_2_1">Principles of Consolidation</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ConsolidationPolicyTextBlock" xlink:to="us-gaap_ConsolidationPolicyTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ContractualObligation" xlink:label="us-gaap_ContractualObligation"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ContractualObligation_lbl" xml:lang="en-US" id="id_11438736_2434912_1_1">Contractual Obligation</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ContractualObligation_lbl" xml:lang="en-US" id="id_11438736_2434912_2_1">Aggregate payment obligation</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ContractualObligation" xlink:to="us-gaap_ContractualObligation_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_ContractualObligationsTerm" xlink:label="sndx_ContractualObligationsTerm"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_ContractualObligationsTerm_lbl" xml:lang="en-US" id="id_11438736_2634682_3_1">Contractual obligations term.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_ContractualObligationsTerm_lbl" xml:lang="en-US" id="id_11438736_2634682_1_1">Contractual Obligations Term</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_ContractualObligationsTerm_lbl" xml:lang="en-US" id="id_11438736_2634682_2_1">Period of contractual obligation</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_ContractualObligationsTerm" xlink:to="sndx_ContractualObligationsTerm_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConversionOfStockAmountConverted1" xlink:label="us-gaap_ConversionOfStockAmountConverted1"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ConversionOfStockAmountConverted1_lbl" xml:lang="en-US" id="id_11438736_2435358_1_1">Conversion of Stock, Amount Converted</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ConversionOfStockAmountConverted1_lbl" xml:lang="en-US" id="id_11438736_2435358_2_1">Conversion of preferred stock to common stock upon closing of initial public offering</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ConversionOfStockAmountConverted1" xlink:to="us-gaap_ConversionOfStockAmountConverted1_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_lbl" xml:lang="en-US" id="id_11438736_2435569_1_1">Convertible Preferred Stock, Shares Issued upon Conversion</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_lbl" xml:lang="en-US" id="id_11438736_2435569_2_1">Convertible preferred stock converted in to common stock</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion" xlink:to="us-gaap_ConvertiblePreferredStockSharesIssuedUponConversion_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_CorporateBondsAndCommercialPaperMember" xlink:label="sndx_CorporateBondsAndCommercialPaperMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_CorporateBondsAndCommercialPaperMember_lbl" xml:lang="en-US" id="id_11438736_2709323_3_1">Corporate bonds and commercial paper.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_CorporateBondsAndCommercialPaperMember_lbl" xml:lang="en-US" id="id_11438736_2709323_1_1">Corporate Bonds And Commercial Paper [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_CorporateBondsAndCommercialPaperMember_lbl" xml:lang="en-US" id="id_11438736_2709323_2_1">Corporate Bonds and Commercial Paper [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_CorporateBondsAndCommercialPaperMember" xlink:to="sndx_CorporateBondsAndCommercialPaperMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="us-gaap_CorporateDebtSecuritiesMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CorporateDebtSecuritiesMember_lbl" xml:lang="en-US" id="id_11438736_2434944_1_1">Corporate Debt Securities [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CorporateDebtSecuritiesMember_lbl" xml:lang="en-US" id="id_11438736_2434944_2_1">Corporate Bonds [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CorporateDebtSecuritiesMember" xlink:to="us-gaap_CorporateDebtSecuritiesMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_CostsRelatedToCostSharingCollaborations" xlink:label="sndx_CostsRelatedToCostSharingCollaborations"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_CostsRelatedToCostSharingCollaborations_lbl" xml:lang="en-US" id="id_11438736_2537011_3_1">Costs related to cost sharing collaborations.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_CostsRelatedToCostSharingCollaborations_lbl" xml:lang="en-US" id="id_11438736_2537011_1_1">Costs Related To Cost Sharing Collaborations</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_CostsRelatedToCostSharingCollaborations_lbl" xml:lang="en-US" id="id_11438736_2537011_2_1">External costs related to cost-sharing collaborations</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_CostsRelatedToCostSharingCollaborations" xlink:to="sndx_CostsRelatedToCostSharingCollaborations_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CounterpartyNameAxis" xlink:label="us-gaap_CounterpartyNameAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CounterpartyNameAxis_lbl" xml:lang="en-US" id="id_11438736_2435412_1_1">Counterparty Name [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CounterpartyNameAxis_lbl" xml:lang="en-US" id="id_11438736_2435412_2_1">Counterparty Name [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CounterpartyNameAxis" xlink:to="us-gaap_CounterpartyNameAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_CowenAndCompanyLLCMember" xlink:label="sndx_CowenAndCompanyLLCMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xml:lang="en-US" id="id_11438736_2705971_3_1">Cowen and Company LLC.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xml:lang="en-US" id="id_11438736_2705971_1_1">Cowen And Company LLC [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_CowenAndCompanyLLCMember_lbl" xml:lang="en-US" id="id_11438736_2705971_2_1">Cowen and Company LLC [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_CowenAndCompanyLLCMember" xlink:to="sndx_CowenAndCompanyLLCMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_lbl" xml:lang="en-US" id="id_11438736_2435156_1_1">Cumulative Effect of New Accounting Principle in Period of Adoption</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_lbl" xml:lang="en-US" id="id_11438736_2435156_2_1">Cumulative effect adjustment of adoption ASU 2016-09</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:to="us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="dei_CurrentFiscalYearEndDate"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xml:lang="en-US" id="id_11438736_2435642_1_1">Current Fiscal Year End Date</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="dei_CurrentFiscalYearEndDate_lbl" xml:lang="en-US" id="id_11438736_2435642_2_1">Current Fiscal Year End Date</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_CurrentFiscalYearEndDate" xlink:to="dei_CurrentFiscalYearEndDate_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscount" xlink:label="us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DebtInstrumentUnamortizedDiscount_lbl" xml:lang="en-US" id="id_11438736_2435693_1_1">Debt Instrument, Unamortized Discount</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_DebtInstrumentUnamortizedDiscount_lbl" xml:lang="en-US" id="id_11438736_2435693_2_1">Initial fair value of derivative recorded as debt discount</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_DebtInstrumentUnamortizedDiscount" xlink:to="us-gaap_DebtInstrumentUnamortizedDiscount_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract_lbl" xml:lang="en-US" id="id_11438736_2435733_1_1">Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_DeferredCostsCapitalizedPrepaidAndOtherAssetsDisclosureAbstract_lbl" xml:lang="en-US" id="id_11438736_2435733_2_1">Deferred Costs, Capitalized, Prepaid, and Other Assets Disclosure [Abstract]</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="dei_EntityCommonStockSharesOutstanding"/>
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    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityFilerCategory" xlink:label="dei_EntityFilerCategory"/>
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    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityRegistrantName" xlink:label="dei_EntityRegistrantName"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_EntityRegistrantName_lbl" xml:lang="en-US" id="id_11438736_2437699_1_1">Entity Registrant Name</label>
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    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_EntityRegistrantName" xlink:to="dei_EntityRegistrantName_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityAbstract" xlink:label="us-gaap_EquityAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EquityAbstract_lbl" xml:lang="en-US" id="id_11438736_2437558_1_1">Equity [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EquityAbstract_lbl" xml:lang="en-US" id="id_11438736_2437558_2_1">Equity [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EquityAbstract" xlink:to="us-gaap_EquityAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="us-gaap_EquityComponentDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_EquityComponentDomain_lbl" xml:lang="en-US" id="id_11438736_2437711_1_1">Equity Component [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_EquityComponentDomain_lbl" xml:lang="en-US" id="id_11438736_2437711_2_1">Equity Component [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_EquityComponentDomain" xlink:to="us-gaap_EquityComponentDomain_lbl"/>
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    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities" xlink:label="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities_lbl" xml:lang="en-US" id="id_11438736_2515090_3_1">Fair value adjustment of warrants and derivative liabilities.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities_lbl" xml:lang="en-US" id="id_11438736_2515090_1_1">Fair Value Adjustment Of Warrants And Derivative Liabilities</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities_lbl" xml:lang="en-US" id="id_11438736_2515090_14_1">Change in fair value of warrants and derivative</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities" xlink:to="sndx_FairValueAdjustmentOfWarrantsAndDerivativeLiabilities_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xml:lang="en-US" id="id_11438736_2438025_1_1">Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl" xml:lang="en-US" id="id_11438736_2438025_2_1">Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xml:lang="en-US" id="id_11438736_2438026_1_1">Fair Value Measurements, Recurring and Nonrecurring [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl" xml:lang="en-US" id="id_11438736_2438026_2_1">Fair Value Measurements, Recurring and Nonrecurring [Table]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:to="us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xml:lang="en-US" id="id_11438736_2438028_1_1">Fair Value, Hierarchy [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl" xml:lang="en-US" id="id_11438736_2438028_2_1">Fair Value, Hierarchy [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:to="us-gaap_FairValueByFairValueHierarchyLevelAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByLiabilityClassAxis" xlink:label="us-gaap_FairValueByLiabilityClassAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueByLiabilityClassAxis_lbl" xml:lang="en-US" id="id_11438736_2437791_1_1">Liability Class [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueByLiabilityClassAxis_lbl" xml:lang="en-US" id="id_11438736_2437791_2_1">Liability Class [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueByLiabilityClassAxis" xlink:to="us-gaap_FairValueByLiabilityClassAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="us-gaap_FairValueByMeasurementBasisAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueByMeasurementBasisAxis_lbl" xml:lang="en-US" id="id_11438736_2438323_1_1">Measurement Basis [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueByMeasurementBasisAxis_lbl" xml:lang="en-US" id="id_11438736_2438323_2_1">Measurement Basis [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueByMeasurementBasisAxis" xlink:to="us-gaap_FairValueByMeasurementBasisAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueDisclosureItemAmountsDomain_lbl" xml:lang="en-US" id="id_11438736_2438041_1_1">Fair Value Measurement [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueDisclosureItemAmountsDomain_lbl" xml:lang="en-US" id="id_11438736_2438041_2_1">Fair Value Measurement [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueDisclosureItemAmountsDomain" xlink:to="us-gaap_FairValueDisclosureItemAmountsDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueDisclosuresAbstract" xlink:label="us-gaap_FairValueDisclosuresAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueDisclosuresAbstract_lbl" xml:lang="en-US" id="id_11438736_2438044_1_1">Fair Value Disclosures [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueDisclosuresAbstract_lbl" xml:lang="en-US" id="id_11438736_2438044_2_1">Fair Value Disclosures [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueDisclosuresAbstract" xlink:to="us-gaap_FairValueDisclosuresAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="us-gaap_FairValueInputsLevel1Member"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xml:lang="en-US" id="id_11438736_2437665_1_1">Fair Value, Inputs, Level 1 [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueInputsLevel1Member_lbl" xml:lang="en-US" id="id_11438736_2437665_2_1">Quoted Prices in Active Markets Level 1 [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueInputsLevel1Member" xlink:to="us-gaap_FairValueInputsLevel1Member_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="us-gaap_FairValueInputsLevel2Member"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xml:lang="en-US" id="id_11438736_2439008_1_1">Fair Value, Inputs, Level 2 [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueInputsLevel2Member_lbl" xml:lang="en-US" id="id_11438736_2439008_2_1">Significant Other Observable Inputs Level 2 [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueInputsLevel2Member" xlink:to="us-gaap_FairValueInputsLevel2Member_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="us-gaap_FairValueInputsLevel3Member"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueInputsLevel3Member_lbl" xml:lang="en-US" id="id_11438736_2438072_1_1">Fair Value, Inputs, Level 3 [Member]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_lbl" xml:lang="en-US" id="id_11438736_2438353_2_1">Fair Value, Liabilities Measured on Recurring Basis, Unobservable Input Reconciliation [Table]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2437864_2_1">Fair Value Measurements</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementInputsDisclosureTextBlock" xlink:to="us-gaap_FairValueMeasurementInputsDisclosureTextBlock_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl" xml:lang="en-US" id="id_11438736_2438100_2_1">Fair Value Hierarchy [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:to="us-gaap_FairValueMeasurementsFairValueHierarchyDomain_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xml:lang="en-US" id="id_11438736_2468726_3_1">Fair Value Measurements [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xml:lang="en-US" id="id_11438736_2468726_1_1">Fair Value Measurements [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_FairValueMeasurementsLineItems_lbl" xml:lang="en-US" id="id_11438736_2468726_2_1">Fair Value Measurements [Line Items]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_FairValueMeasurementsLineItems" xlink:to="sndx_FairValueMeasurementsLineItems_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FairValueMeasurementsTable_lbl" xml:lang="en-US" id="id_11438736_2649099_3_1">Fair Value Measurements [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_FairValueMeasurementsTable_lbl" xml:lang="en-US" id="id_11438736_2649099_1_1">Fair Value Measurements [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_FairValueMeasurementsTable_lbl" xml:lang="en-US" id="id_11438736_2649099_2_1">Fair Value Measurements [Table]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_FairValueMeasurementsTable" xlink:to="sndx_FairValueMeasurementsTable_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xml:lang="en-US" id="id_11438736_2594373_3_1">Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Change In Fair Value</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xml:lang="en-US" id="id_11438736_2594373_1_1">Fair Value Measurement With Unobservable Inputs Reconciliations Recurring Basis Liability Change In Fair Value</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl" xml:lang="en-US" id="id_11438736_2594373_2_1">Change in fair value</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue" xlink:to="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityChangeInFairValue_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityReclassificationToEquity_lbl" xml:lang="en-US" id="id_11438736_2594219_3_1">Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Reclassification To Equity</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityReclassificationToEquity_lbl" xml:lang="en-US" id="id_11438736_2594219_1_1">Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Reclassification To Equity</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="sndx_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityReclassificationToEquity_lbl" xml:lang="en-US" id="id_11438736_2594219_14_1">Reclassification</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xml:lang="en-US" id="id_11438736_2438114_1_1">Fair Value, Measurement with Unobservable Inputs Reconciliations, Recurring Basis, Liability Value</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_lbl" xml:lang="en-US" id="id_11438736_2438114_4_1">Ending Balance</label>
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    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_FollowOnPublicOfferingMember" xlink:label="sndx_FollowOnPublicOfferingMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_FollowOnPublicOfferingMember_lbl" xml:lang="en-US" id="id_11438736_2704729_3_1">Follow On Public Offering [Member]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncomeStatementAbstract_lbl" xml:lang="en-US" id="id_11438736_2440732_2_1">Income Statement [Abstract]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncomeStatementLocationDomain_lbl" xml:lang="en-US" id="id_11438736_2440347_2_1">Income Statement Location [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncomeStatementLocationDomain" xlink:to="us-gaap_IncomeStatementLocationDomain_lbl"/>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayable" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xml:lang="en-US" id="id_11438736_2440761_1_1">Increase (Decrease) in Accounts Payable</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInAccountsPayable_lbl" xml:lang="en-US" id="id_11438736_2440761_2_1">Accounts payable</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities" xlink:label="us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities"/>
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    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities" xlink:to="us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_IncreaseDecreaseInContractualObligation" xlink:label="sndx_IncreaseDecreaseInContractualObligation"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xml:lang="en-US" id="id_11438736_2538617_3_1">Increase decrease in contractual obligation.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xml:lang="en-US" id="id_11438736_2538617_1_1">Increase Decrease In Contractual Obligation</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_IncreaseDecreaseInContractualObligation_lbl" xml:lang="en-US" id="id_11438736_2538617_2_1">Increase in contractual obligation</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_IncreaseDecreaseInContractualObligation" xlink:to="sndx_IncreaseDecreaseInContractualObligation_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredCompensation" xlink:label="us-gaap_IncreaseDecreaseInDeferredCompensation"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInDeferredCompensation_lbl" xml:lang="en-US" id="id_11438736_2440772_1_1">Increase (Decrease) in Deferred Compensation</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInDeferredCompensation_lbl" xml:lang="en-US" id="id_11438736_2440772_2_1">Reduction in employee payroll</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInDeferredCompensation" xlink:to="us-gaap_IncreaseDecreaseInDeferredCompensation_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:label="us-gaap_IncreaseDecreaseInDeferredRevenue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInDeferredRevenue_lbl" xml:lang="en-US" id="id_11438736_2440203_1_1">Increase (Decrease) in Deferred Revenue</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInDeferredRevenue_lbl" xml:lang="en-US" id="id_11438736_2440203_2_1">Deferred revenue</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInDeferredRevenue" xlink:to="us-gaap_IncreaseDecreaseInDeferredRevenue_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xml:lang="en-US" id="id_11438736_2440238_1_1">Increase (Decrease) in Operating Capital [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl" xml:lang="en-US" id="id_11438736_2440238_2_1">Changes in operating assets and liabilities:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract" xlink:to="us-gaap_IncreaseDecreaseInOperatingCapitalAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xml:lang="en-US" id="id_11438736_2440814_1_1">Increase (Decrease) in Prepaid Expense and Other Assets</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl" xml:lang="en-US" id="id_11438736_2440814_14_1">Prepaid expenses and other assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:to="us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_IncreaseDecreaseInResearchAndDevelopmentExpense" xlink:label="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2579440_3_1">Increase Decrease In Research And Development Expense</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2579440_1_1">Increase Decrease In Research And Development Expense</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2579440_2_1">Cost-sharing collaborations recorded as a reduction to research and development expense</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense" xlink:to="sndx_IncreaseDecreaseInResearchAndDevelopmentExpense_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IncreaseDecreaseInRestrictedCash" xlink:label="us-gaap_IncreaseDecreaseInRestrictedCash"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IncreaseDecreaseInRestrictedCash_lbl" xml:lang="en-US" id="id_11438736_2440822_1_1">Increase (Decrease) in Restricted Cash</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_IncreaseDecreaseInRestrictedCash_lbl" xml:lang="en-US" id="id_11438736_2440822_14_1">Changes in restricted cash</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IncreaseDecreaseInRestrictedCash" xlink:to="us-gaap_IncreaseDecreaseInRestrictedCash_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet" xlink:label="us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_InterestIncomeExpenseNonoperatingNet_lbl" xml:lang="en-US" id="id_11438736_2441204_1_1">Interest Income (Expense), Nonoperating, Net</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_InterestIncomeExpenseNonoperatingNet_lbl" xml:lang="en-US" id="id_11438736_2441204_2_1">Interest income, net</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_InterestIncomeExpenseNonoperatingNet" xlink:to="us-gaap_InterestIncomeExpenseNonoperatingNet_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InterestPaid" xlink:label="us-gaap_InterestPaid"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_InterestPaid_lbl" xml:lang="en-US" id="id_11438736_2441446_1_1">Interest Paid</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_InterestPaid_lbl" xml:lang="en-US" id="id_11438736_2441446_2_1">Interest paid</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_InterestPaid" xlink:to="us-gaap_InterestPaid_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InterestReceivableCurrent" xlink:label="us-gaap_InterestReceivableCurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_InterestReceivableCurrent_lbl" xml:lang="en-US" id="id_11438736_2441459_1_1">Interest Receivable, Current</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_InterestReceivableCurrent_lbl" xml:lang="en-US" id="id_11438736_2441459_2_1">Interest receivable on short-term investments</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_InterestReceivableCurrent" xlink:to="us-gaap_InterestReceivableCurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract" xlink:label="us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract_lbl" xml:lang="en-US" id="id_11438736_2441046_1_1">Investment Company, Capital Share Transactions [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract_lbl" xml:lang="en-US" id="id_11438736_2441046_2_1">Investment Company, Capital Share Transactions [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract" xlink:to="us-gaap_InvestmentCompanyCapitalShareTransactionsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_IPOMember" xlink:label="us-gaap_IPOMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_IPOMember_lbl" xml:lang="en-US" id="id_11438736_2441341_1_1">IPO [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_IPOMember_lbl" xml:lang="en-US" id="id_11438736_2441341_2_1">Initial Public Offering [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_IPOMember" xlink:to="us-gaap_IPOMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses" xlink:label="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses_lbl" xml:lang="en-US" id="id_11438736_2541336_3_1">Issuance costs included in accounts payable and accrued expenses.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses_lbl" xml:lang="en-US" id="id_11438736_2541336_1_1">Issuance Costs Included In Accounts Payable And Accrued Expenses</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses_lbl" xml:lang="en-US" id="id_11438736_2541336_2_1">Issuance costs included in accounts payable and accrued expenses</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses" xlink:to="sndx_IssuanceCostsIncludedInAccountsPayableAndAccruedExpenses_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_KyowaHakkoKirinCoLtdMember" xlink:label="sndx_KyowaHakkoKirinCoLtdMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_KyowaHakkoKirinCoLtdMember_lbl" xml:lang="en-US" id="id_11438736_2669132_3_1">Kyowa Hakko Kirin Co Ltd</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_KyowaHakkoKirinCoLtdMember_lbl" xml:lang="en-US" id="id_11438736_2669132_1_1">Kyowa Hakko Kirin Co Ltd [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_KyowaHakkoKirinCoLtdMember_lbl" xml:lang="en-US" id="id_11438736_2669132_2_1">Kyowa Hakko Kirin Co., Ltd. [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_KyowaHakkoKirinCoLtdMember" xlink:to="sndx_KyowaHakkoKirinCoLtdMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="dei_LegalEntityAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_LegalEntityAxis_lbl" xml:lang="en-US" id="id_11438736_2441826_1_1">Legal Entity [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="dei_LegalEntityAxis_lbl" xml:lang="en-US" id="id_11438736_2441826_2_1">Legal Entity [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_LegalEntityAxis" xlink:to="dei_LegalEntityAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Liabilities" xlink:label="us-gaap_Liabilities"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_Liabilities_lbl" xml:lang="en-US" id="id_11438736_2442110_1_1">Liabilities</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_Liabilities_lbl" xml:lang="en-US" id="id_11438736_2442110_6_1">Total liabilities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_Liabilities" xlink:to="us-gaap_Liabilities_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2442111_1_1">Liabilities and Equity</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_LiabilitiesAndStockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2442111_6_1">Total liabilities and stockholders' equity</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesAndStockholdersEquity" xlink:to="us-gaap_LiabilitiesAndStockholdersEquity_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LiabilitiesAndStockholdersEquityAbstract_lbl" xml:lang="en-US" id="id_11438736_2441888_2_1">LIABILITIES AND STOCKHOLDERS' EQUITY</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_LiabilitiesCurrent_lbl" xml:lang="en-US" id="id_11438736_2442451_6_1">Total current liabilities</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="us-gaap_LiabilitiesCurrentAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xml:lang="en-US" id="id_11438736_2442113_1_1">Liabilities, Current [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LiabilitiesCurrentAbstract_lbl" xml:lang="en-US" id="id_11438736_2442113_2_1">Current liabilities:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LiabilitiesCurrentAbstract" xlink:to="us-gaap_LiabilitiesCurrentAbstract_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2442341_1_1">Liabilities, Noncurrent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_LiabilitiesNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2442341_6_1">Total long-term liabilities</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LiabilitiesNoncurrentAbstract_lbl" xml:lang="en-US" id="id_11438736_2441896_1_1">Liabilities, Noncurrent [Abstract]</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_LicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2659749_3_1">License Agreement [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_LicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2659749_1_1">License Agreement [Member]</label>
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    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_LicenseAgreementMember" xlink:to="sndx_LicenseAgreementMember_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_LicenseAgreementsExpirationPeriod_lbl" xml:lang="en-US" id="id_11438736_2632213_3_1">License agreements expiration period.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_LicenseAgreementsExpirationPeriod_lbl" xml:lang="en-US" id="id_11438736_2632213_1_1">License Agreements Expiration Period</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LicensesRevenue" xlink:label="us-gaap_LicensesRevenue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LicensesRevenue_lbl" xml:lang="en-US" id="id_11438736_2441941_1_1">Licenses Revenue</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LicensesRevenue_lbl" xml:lang="en-US" id="id_11438736_2441941_2_1">License fees</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LongtermDebtTypeAxis_lbl" xml:lang="en-US" id="id_11438736_2442761_1_1">Long-term Debt, Type [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LongtermDebtTypeAxis_lbl" xml:lang="en-US" id="id_11438736_2442761_2_1">Long-term Debt, Type [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LongtermDebtTypeAxis" xlink:to="us-gaap_LongtermDebtTypeAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_LongtermDebtTypeDomain" xlink:label="us-gaap_LongtermDebtTypeDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_LongtermDebtTypeDomain_lbl" xml:lang="en-US" id="id_11438736_2443186_1_1">Long-term Debt, Type [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_LongtermDebtTypeDomain_lbl" xml:lang="en-US" id="id_11438736_2443186_2_1">Long-term Debt, Type [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_LongtermDebtTypeDomain" xlink:to="us-gaap_LongtermDebtTypeDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_lbl" xml:lang="en-US" id="id_11438736_2443024_1_1">Major Types of Debt and Equity Securities [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_lbl" xml:lang="en-US" id="id_11438736_2443024_2_1">Major Types of Debt and Equity Securities [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_lbl" xml:lang="en-US" id="id_11438736_2443354_1_1">Major Types of Debt and Equity Securities [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_lbl" xml:lang="en-US" id="id_11438736_2443354_2_1">Major Types of Debt and Equity Securities [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:to="us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MarketableSecuritiesUnrealizedGainLoss" xlink:label="us-gaap_MarketableSecuritiesUnrealizedGainLoss"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MarketableSecuritiesUnrealizedGainLoss_lbl" xml:lang="en-US" id="id_11438736_2442846_1_1">Marketable Securities, Unrealized Gain (Loss)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MarketableSecuritiesUnrealizedGainLoss_lbl" xml:lang="en-US" id="id_11438736_2442846_2_1">Unrealized losses on short-term investments</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MarketableSecuritiesUnrealizedGainLoss" xlink:to="us-gaap_MarketableSecuritiesUnrealizedGainLoss_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MaximumMember" xlink:label="us-gaap_MaximumMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MaximumMember_lbl" xml:lang="en-US" id="id_11438736_2442860_1_1">Maximum [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MaximumMember_lbl" xml:lang="en-US" id="id_11438736_2442860_2_1">Maximum [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MaximumMember" xlink:to="us-gaap_MaximumMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_MilestonePaymentDue" xlink:label="sndx_MilestonePaymentDue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_MilestonePaymentDue_lbl" xml:lang="en-US" id="id_11438736_2549807_3_1">Milestone payment due.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_MilestonePaymentDue_lbl" xml:lang="en-US" id="id_11438736_2549807_1_1">Milestone Payment Due</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_MilestonePaymentDue_lbl" xml:lang="en-US" id="id_11438736_2549807_2_1">Milestone payment payable</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_MilestonePaymentDue" xlink:to="sndx_MilestonePaymentDue_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_MinimumMember" xlink:label="us-gaap_MinimumMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_MinimumMember_lbl" xml:lang="en-US" id="id_11438736_2443301_1_1">Minimum [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_MinimumMember_lbl" xml:lang="en-US" id="id_11438736_2443301_2_1">Minimum [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_MinimumMember" xlink:to="us-gaap_MinimumMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_NatureOfBusinessLineItems" xlink:label="sndx_NatureOfBusinessLineItems"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_NatureOfBusinessLineItems_lbl" xml:lang="en-US" id="id_11438736_2470725_3_1">Nature of Business [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_NatureOfBusinessLineItems_lbl" xml:lang="en-US" id="id_11438736_2470725_1_1">Nature Of Business [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_NatureOfBusinessLineItems_lbl" xml:lang="en-US" id="id_11438736_2470725_2_1">Nature of Business [Line Items]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_NatureOfBusinessLineItems" xlink:to="sndx_NatureOfBusinessLineItems_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_NatureOfBusinessTable" xlink:label="sndx_NatureOfBusinessTable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_NatureOfBusinessTable_lbl" xml:lang="en-US" id="id_11438736_2648734_3_1">Nature Of Business [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_NatureOfBusinessTable_lbl" xml:lang="en-US" id="id_11438736_2648734_1_1">Nature Of Business [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_NatureOfBusinessTable_lbl" xml:lang="en-US" id="id_11438736_2648734_2_1">Nature Of Business [Table]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_NatureOfBusinessTable" xlink:to="sndx_NatureOfBusinessTable_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NatureOfOperations" xlink:label="us-gaap_NatureOfOperations"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NatureOfOperations_lbl" xml:lang="en-US" id="id_11438736_2443490_1_1">Nature of Operations [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NatureOfOperations_lbl" xml:lang="en-US" id="id_11438736_2443490_2_1">Nature of Business</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NatureOfOperations" xlink:to="us-gaap_NatureOfOperations_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_lbl" xml:lang="en-US" id="id_11438736_2444055_1_1">Net Cash Provided by (Used in) Financing Activities, Continuing Operations</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_lbl" xml:lang="en-US" id="id_11438736_2444055_6_1">Net cash provided by financing activities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperations_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443641_1_1">Net Cash Provided by (Used in) Financing Activities, Continuing Operations [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443641_2_1">CASH FLOWS FROM FINANCING ACTIVITIES:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInFinancingActivitiesContinuingOperationsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_lbl" xml:lang="en-US" id="id_11438736_2443502_1_1">Net Cash Provided by (Used in) Investing Activities, Continuing Operations</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_lbl" xml:lang="en-US" id="id_11438736_2443502_6_1">Net cash used in investing activities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperations_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443643_1_1">Net Cash Provided by (Used in) Investing Activities, Continuing Operations [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443643_2_1">CASH FLOWS FROM INVESTING ACTIVITIES:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInInvestingActivitiesContinuingOperationsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperations"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443504_1_1">Net Cash Provided by (Used in) Operating Activities, Continuing Operations [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract_lbl" xml:lang="en-US" id="id_11438736_2443504_2_1">CASH FLOWS FROM OPERATING ACTIVITIES:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract" xlink:to="us-gaap_NetCashProvidedByUsedInOperatingActivitiesContinuingOperationsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="us-gaap_NetIncomeLoss"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetIncomeLoss_lbl" xml:lang="en-US" id="id_11438736_2444061_1_1">Net Income (Loss) Attributable to Parent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetIncomeLoss_lbl" xml:lang="en-US" id="id_11438736_2444061_2_1">Net loss</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetIncomeLoss_lbl" xml:lang="en-US" id="id_11438736_2444061_6_1">Net loss</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetIncomeLoss" xlink:to="us-gaap_NetIncomeLoss_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetIncomeLossAbstract" xlink:label="us-gaap_NetIncomeLossAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetIncomeLossAbstract_lbl" xml:lang="en-US" id="id_11438736_2443512_1_1">Net Income (Loss) Attributable to Parent [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetIncomeLossAbstract_lbl" xml:lang="en-US" id="id_11438736_2443512_2_1">Numerator-basic and diluted:</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xml:lang="en-US" id="id_11438736_2443515_1_1">Net Income (Loss) Available to Common Stockholders, Basic</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xml:lang="en-US" id="id_11438736_2443515_2_1">Net loss attributable to common stockholders</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl" xml:lang="en-US" id="id_11438736_2443515_6_1">Net loss attributable to common stockholders-basic and diluted</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic" xlink:to="us-gaap_NetIncomeLossAvailableToCommonStockholdersBasic_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2444191_1_1">New Accounting Pronouncements, Policy [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2444191_2_1">Recently Issued and Adopted Accounting Pronouncements</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock" xlink:to="us-gaap_NewAccountingPronouncementsPolicyPolicyTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:label="us-gaap_NoncashInvestingAndFinancingItemsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NoncashInvestingAndFinancingItemsAbstract_lbl" xml:lang="en-US" id="id_11438736_2444090_1_1">Noncash Investing and Financing Items [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NoncashInvestingAndFinancingItemsAbstract_lbl" xml:lang="en-US" id="id_11438736_2444090_2_1">SUPPLEMENTAL DISCLOSURES OF NONCASH INVESTING AND FINANCING ACTIVITIES:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NoncashInvestingAndFinancingItemsAbstract" xlink:to="us-gaap_NoncashInvestingAndFinancingItemsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NonoperatingIncomeExpense" xlink:label="us-gaap_NonoperatingIncomeExpense"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NonoperatingIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2444227_1_1">Nonoperating Income (Expense)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_NonoperatingIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2444227_6_1">Total other income (expense)</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NonoperatingIncomeExpense" xlink:to="us-gaap_NonoperatingIncomeExpense_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_NonoperatingIncomeExpenseAbstract" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract_lbl" xml:lang="en-US" id="id_11438736_2444228_1_1">Nonoperating Income (Expense) [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_NonoperatingIncomeExpenseAbstract_lbl" xml:lang="en-US" id="id_11438736_2444228_2_1">Other income (expense):</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_NonoperatingIncomeExpenseAbstract" xlink:to="us-gaap_NonoperatingIncomeExpenseAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_NonRefundableUpFrontPaymentReceived" xlink:label="sndx_NonRefundableUpFrontPaymentReceived"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_NonRefundableUpFrontPaymentReceived_lbl" xml:lang="en-US" id="id_11438736_2530000_3_1">Non Refundable Up Front Payment Received</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_NonRefundableUpFrontPaymentReceived_lbl" xml:lang="en-US" id="id_11438736_2530000_1_1">Non Refundable Up Front Payment Received</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_NonRefundableUpFrontPaymentReceived_lbl" xml:lang="en-US" id="id_11438736_2530000_2_1">Up-front license fee received</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_NonRefundableUpFrontPaymentReceived" xlink:to="sndx_NonRefundableUpFrontPaymentReceived_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_NumberOfExecutivesOptionsVested" xlink:label="sndx_NumberOfExecutivesOptionsVested"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_NumberOfExecutivesOptionsVested_lbl" xml:lang="en-US" id="id_11438736_2629669_3_1">Number of executives options vested.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_NumberOfExecutivesOptionsVested_lbl" xml:lang="en-US" id="id_11438736_2629669_1_1">Number Of Executives Options Vested</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_NumberOfExecutivesOptionsVested_lbl" xml:lang="en-US" id="id_11438736_2629669_2_1">Number of company executives</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_NumberOfExecutivesOptionsVested" xlink:to="sndx_NumberOfExecutivesOptionsVested_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_NumberOfRevenueArrangement" xlink:label="sndx_NumberOfRevenueArrangement"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_NumberOfRevenueArrangement_lbl" xml:lang="en-US" id="id_11438736_2899680_3_1">Number of revenue arrangement.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_NumberOfRevenueArrangement_lbl" xml:lang="en-US" id="id_11438736_2899680_1_1">Number of Revenue Arrangement</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_NumberOfRevenueArrangement_lbl" xml:lang="en-US" id="id_11438736_2899680_2_1">Number of revenue arrangement</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_NumberOfRevenueArrangement" xlink:to="sndx_NumberOfRevenueArrangement_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingExpenses" xlink:label="us-gaap_OperatingExpenses"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingExpenses_lbl" xml:lang="en-US" id="id_11438736_2443964_1_1">Operating Expenses</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_OperatingExpenses_lbl" xml:lang="en-US" id="id_11438736_2443964_6_1">Total operating expenses</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingExpenses" xlink:to="us-gaap_OperatingExpenses_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="us-gaap_OperatingExpensesAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xml:lang="en-US" id="id_11438736_2444430_1_1">Operating Expenses [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OperatingExpensesAbstract_lbl" xml:lang="en-US" id="id_11438736_2444430_2_1">Operating expenses:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingExpensesAbstract" xlink:to="us-gaap_OperatingExpensesAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="us-gaap_OperatingIncomeLoss"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xml:lang="en-US" id="id_11438736_2444189_1_1">Operating Income (Loss)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_OperatingIncomeLoss_lbl" xml:lang="en-US" id="id_11438736_2444189_6_1">Loss from operations</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OperatingIncomeLoss" xlink:to="us-gaap_OperatingIncomeLoss_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xml:lang="en-US" id="id_11438736_2444466_1_1">Organization, Consolidation and Presentation of Financial Statements [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl" xml:lang="en-US" id="id_11438736_2444466_2_1">Organization, Consolidation and Presentation of Financial Statements [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract" xlink:to="us-gaap_OrganizationConsolidationAndPresentationOfFinancialStatementsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherAccruedLiabilitiesCurrent" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent_lbl" xml:lang="en-US" id="id_11438736_2444315_1_1">Other Accrued Liabilities, Current</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherAccruedLiabilitiesCurrent_lbl" xml:lang="en-US" id="id_11438736_2444315_2_1">Other</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherAccruedLiabilitiesCurrent" xlink:to="us-gaap_OtherAccruedLiabilitiesCurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="us-gaap_OtherAssetsNoncurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444650_1_1">Other Assets, Noncurrent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherAssetsNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444650_2_1">Other assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherAssetsNoncurrent" xlink:to="us-gaap_OtherAssetsNoncurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_OtherAssetsPolicyTextBlock" xlink:label="sndx_OtherAssetsPolicyTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_OtherAssetsPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2767440_3_1">Disclosure of accounting policy for other noncurrent assets.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_OtherAssetsPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2767440_1_1">Other Assets [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_OtherAssetsPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2767440_2_1">Other Assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_OtherAssetsPolicyTextBlock" xlink:to="sndx_OtherAssetsPolicyTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl" xml:lang="en-US" id="id_11438736_2444704_1_1">Other Comprehensive Income (Loss), Net of Tax [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl" xml:lang="en-US" id="id_11438736_2444704_2_1">Other comprehensive (loss) income:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract" xlink:to="us-gaap_OtherComprehensiveIncomeLossNetOfTaxPeriodIncreaseDecreaseAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2444921_1_1">Other Comprehensive Income (Loss), Unrealized Holding Gain (Loss) on Securities Arising During Period, Net of Tax</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl" xml:lang="en-US" id="id_11438736_2444921_2_1">Unrealized (losses) gains on marketable securities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:to="us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherCurrentAssetsTextBlock" xlink:label="us-gaap_OtherCurrentAssetsTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherCurrentAssetsTextBlock_lbl" xml:lang="en-US" id="id_11438736_2444932_1_1">Other Current Assets [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherCurrentAssetsTextBlock_lbl" xml:lang="en-US" id="id_11438736_2444932_2_1">Prepaid Expenses and Other Current Assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherCurrentAssetsTextBlock" xlink:to="us-gaap_OtherCurrentAssetsTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="us-gaap_OtherLiabilitiesNoncurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherLiabilitiesNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444753_1_1">Other Liabilities, Noncurrent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherLiabilitiesNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444753_2_1">Other long-term liabilities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherLiabilitiesNoncurrent" xlink:to="us-gaap_OtherLiabilitiesNoncurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherLongTermDebtNoncurrent" xlink:label="us-gaap_OtherLongTermDebtNoncurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherLongTermDebtNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444554_1_1">Other Long-term Debt, Noncurrent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherLongTermDebtNoncurrent_lbl" xml:lang="en-US" id="id_11438736_2444554_2_1">Success fee payable included in other long-term liabilities</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherLongTermDebtNoncurrent" xlink:to="us-gaap_OtherLongTermDebtNoncurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherNoncashIncomeExpense" xlink:label="us-gaap_OtherNoncashIncomeExpense"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherNoncashIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2445064_1_1">Other Noncash Income (Expense)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_OtherNoncashIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2445064_14_1">Other</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherNoncashIncomeExpense" xlink:to="us-gaap_OtherNoncashIncomeExpense_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="us-gaap_OtherNonoperatingIncomeExpense"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2445068_1_1">Other Nonoperating Income (Expense)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherNonoperatingIncomeExpense_lbl" xml:lang="en-US" id="id_11438736_2445068_2_1">Other income (expense)</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherNonoperatingIncomeExpense" xlink:to="us-gaap_OtherNonoperatingIncomeExpense_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OtherPrepaidExpenseCurrent" xlink:label="us-gaap_OtherPrepaidExpenseCurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xml:lang="en-US" id="id_11438736_2444776_1_1">Other Prepaid Expense, Current</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OtherPrepaidExpenseCurrent_lbl" xml:lang="en-US" id="id_11438736_2444776_2_1">Other</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OtherPrepaidExpenseCurrent" xlink:to="us-gaap_OtherPrepaidExpenseCurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_OverAllotmentOptionMember" xlink:label="us-gaap_OverAllotmentOptionMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_OverAllotmentOptionMember_lbl" xml:lang="en-US" id="id_11438736_2445264_1_1">Over-Allotment Option [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_OverAllotmentOptionMember_lbl" xml:lang="en-US" id="id_11438736_2445264_2_1">Over-Allotment Option [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_OverAllotmentOptionMember" xlink:to="us-gaap_OverAllotmentOptionMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_OvernightInvestmentsAndMoneyMarketFundsMember" xlink:label="sndx_OvernightInvestmentsAndMoneyMarketFundsMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_OvernightInvestmentsAndMoneyMarketFundsMember_lbl" xml:lang="en-US" id="id_11438736_2699820_3_1">Overnight investments and money market funds.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_OvernightInvestmentsAndMoneyMarketFundsMember_lbl" xml:lang="en-US" id="id_11438736_2699820_1_1">Overnight Investments And Money Market Funds [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_OvernightInvestmentsAndMoneyMarketFundsMember_lbl" xml:lang="en-US" id="id_11438736_2699820_2_1">Overnight Investments and Money Market Funds [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_OvernightInvestmentsAndMoneyMarketFundsMember" xlink:to="sndx_OvernightInvestmentsAndMoneyMarketFundsMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PayablesAndAccrualsAbstract" xlink:label="us-gaap_PayablesAndAccrualsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PayablesAndAccrualsAbstract_lbl" xml:lang="en-US" id="id_11438736_2445297_1_1">Payables and Accruals [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PayablesAndAccrualsAbstract_lbl" xml:lang="en-US" id="id_11438736_2445297_2_1">Payables and Accruals [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PayablesAndAccrualsAbstract" xlink:to="us-gaap_PayablesAndAccrualsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsOfStockIssuanceCosts" xlink:label="us-gaap_PaymentsOfStockIssuanceCosts"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PaymentsOfStockIssuanceCosts_lbl" xml:lang="en-US" id="id_11438736_2445738_1_1">Payments of Stock Issuance Costs</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PaymentsOfStockIssuanceCosts_lbl" xml:lang="en-US" id="id_11438736_2445738_2_1">Offering expenses</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PaymentsOfStockIssuanceCosts" xlink:to="us-gaap_PaymentsOfStockIssuanceCosts_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares" xlink:label="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares_lbl" xml:lang="en-US" id="id_11438736_2792480_3_1">Payment specified percentage of gross proceeds from sale of shares.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares_lbl" xml:lang="en-US" id="id_11438736_2792480_1_1">Payment Specified Percentage Of Gross Proceeds From Sale Of Shares</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares_lbl" xml:lang="en-US" id="id_11438736_2792480_2_1">Payment percentage of gross proceeds from sales agreement</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares" xlink:to="sndx_PaymentSpecifiedPercentageOfGrossProceedsFromSaleOfShares_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_lbl" xml:lang="en-US" id="id_11438736_2445544_1_1">Payments to Acquire Property, Plant, and Equipment</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_lbl" xml:lang="en-US" id="id_11438736_2445544_14_1">Purchases of property and equipment</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:to="us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PaymentsToAcquireShortTermInvestments" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments_lbl" xml:lang="en-US" id="id_11438736_2445755_1_1">Payments to Acquire Short-term Investments</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_PaymentsToAcquireShortTermInvestments_lbl" xml:lang="en-US" id="id_11438736_2445755_14_1">Purchases of short-term investments</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PaymentsToAcquireShortTermInvestments" xlink:to="us-gaap_PaymentsToAcquireShortTermInvestments_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_PercentageOfSharesIssuableOnConversionOfWarrant" xlink:label="sndx_PercentageOfSharesIssuableOnConversionOfWarrant"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_PercentageOfSharesIssuableOnConversionOfWarrant_lbl" xml:lang="en-US" id="id_11438736_2781813_3_1">Percentage of shares issuable on conversion of warrant.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_PercentageOfSharesIssuableOnConversionOfWarrant_lbl" xml:lang="en-US" id="id_11438736_2781813_1_1">Percentage Of Shares Issuable On Conversion Of Warrant</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_PercentageOfSharesIssuableOnConversionOfWarrant_lbl" xml:lang="en-US" id="id_11438736_2781813_2_1">Percentage of shares issuable upon conversion of warrant</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_PercentageOfSharesIssuableOnConversionOfWarrant" xlink:to="sndx_PercentageOfSharesIssuableOnConversionOfWarrant_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" xlink:label="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct_lbl" xml:lang="en-US" id="id_11438736_2639107_3_1">Period of termination after first commercial sale of first licensed product.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct_lbl" xml:lang="en-US" id="id_11438736_2639107_1_1">Period of Termination After First Commercial Sale of First Licensed Product</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct_lbl" xml:lang="en-US" id="id_11438736_2639107_2_1">Period of termination after first commercial sale of first licensed product</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct" xlink:to="sndx_PeriodOfTerminationAfterFirstCommercialSaleOfFirstLicensedProduct_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameAxis" xlink:label="us-gaap_PlanNameAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PlanNameAxis_lbl" xml:lang="en-US" id="id_11438736_2445234_1_1">Plan Name [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PlanNameAxis_lbl" xml:lang="en-US" id="id_11438736_2445234_2_1">Plan Name [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PlanNameAxis" xlink:to="us-gaap_PlanNameAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PlanNameDomain" xlink:label="us-gaap_PlanNameDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PlanNameDomain_lbl" xml:lang="en-US" id="id_11438736_2445571_1_1">Plan Name [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PlanNameDomain_lbl" xml:lang="en-US" id="id_11438736_2445571_2_1">Plan Name [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PlanNameDomain" xlink:to="us-gaap_PlanNameDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember_lbl" xml:lang="en-US" id="id_11438736_2445807_1_1">Portion at Fair Value Measurement [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PortionAtFairValueFairValueDisclosureMember_lbl" xml:lang="en-US" id="id_11438736_2445807_2_1">Portion at Fair Value Measurement [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:to="us-gaap_PortionAtFairValueFairValueDisclosureMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PostemploymentBenefitsAbstract" xlink:label="us-gaap_PostemploymentBenefitsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PostemploymentBenefitsAbstract_lbl" xml:lang="en-US" id="id_11438736_2445439_1_1">Postemployment Benefits [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PostemploymentBenefitsAbstract_lbl" xml:lang="en-US" id="id_11438736_2445439_2_1">Postemployment Benefits [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PostemploymentBenefitsAbstract" xlink:to="us-gaap_PostemploymentBenefitsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_PotentialMilestonePaymentsReceivable" xlink:label="sndx_PotentialMilestonePaymentsReceivable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_PotentialMilestonePaymentsReceivable_lbl" xml:lang="en-US" id="id_11438736_2581928_3_1">Potential Milestone Payments Receivable</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_PotentialMilestonePaymentsReceivable_lbl" xml:lang="en-US" id="id_11438736_2581928_1_1">Potential Milestone Payments Receivable</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_PotentialMilestonePaymentsReceivable_lbl" xml:lang="en-US" id="id_11438736_2581928_2_1">Milestone payment receivable upon achievement of development and commercial milestone</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_PotentialMilestonePaymentsReceivable" xlink:to="sndx_PotentialMilestonePaymentsReceivable_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_PotentialMilestonePaymentsUponAchievementOfMilestone" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xml:lang="en-US" id="id_11438736_2503144_3_1">Potential Milestone Payments Upon Achievement Of Milestone</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xml:lang="en-US" id="id_11438736_2503144_1_1">Potential Milestone Payments Upon Achievement Of Milestone</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl" xml:lang="en-US" id="id_11438736_2503144_2_1">Potential milestone payments to be made</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone" xlink:to="sndx_PotentialMilestonePaymentsUponAchievementOfMilestone_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockParOrStatedValuePerShare" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xml:lang="en-US" id="id_11438736_2445650_1_1">Preferred Stock, Par or Stated Value Per Share</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockParOrStatedValuePerShare_lbl" xml:lang="en-US" id="id_11438736_2445650_2_1">Preferred stock, par value</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockParOrStatedValuePerShare" xlink:to="us-gaap_PreferredStockParOrStatedValuePerShare_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockRedemptionAmount" xlink:label="us-gaap_PreferredStockRedemptionAmount"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockSharesIssued_lbl" xml:lang="en-US" id="id_11438736_2445856_1_1">Preferred Stock, Shares Issued</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2445934_1_1">Preferred Stock, Shares Outstanding</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockSharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2445934_2_1">Preferred stock, shares outstanding</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockSharesOutstanding" xlink:to="us-gaap_PreferredStockSharesOutstanding_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PreferredStockValue" xlink:label="us-gaap_PreferredStockValue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PreferredStockValue_lbl" xml:lang="en-US" id="id_11438736_2445656_1_1">Preferred Stock, Value, Issued</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PreferredStockValue_lbl" xml:lang="en-US" id="id_11438736_2445656_2_1">Preferred stock, $0.001 par value, 10,000,000 shares authorized; 0 shares outstanding at September 30, 2017 and December 31, 2016</label>
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    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PreferredStockValue" xlink:to="us-gaap_PreferredStockValue_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" id="id_11438736_2445671_1_1">Prepaid Expense and Other Assets, Current</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" id="id_11438736_2445671_2_1">Prepaid expenses and other current assets</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl" xml:lang="en-US" id="id_11438736_2445671_6_1">Total prepaid expenses and other current assets</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:to="us-gaap_PrepaidExpenseAndOtherAssetsCurrent_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PrepaidInsurance" xlink:label="us-gaap_PrepaidInsurance"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PrepaidInsurance_lbl" xml:lang="en-US" id="id_11438736_2445955_1_1">Prepaid Insurance</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PrepaidInsurance_lbl" xml:lang="en-US" id="id_11438736_2445955_2_1">Prepaid insurance</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PrepaidInsurance" xlink:to="us-gaap_PrepaidInsurance_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:label="us-gaap_ProceedsFromIssuanceInitialPublicOffering"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceInitialPublicOffering_lbl" xml:lang="en-US" id="id_11438736_2446365_1_1">Proceeds from Issuance Initial Public Offering</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromIssuanceInitialPublicOffering_lbl" xml:lang="en-US" id="id_11438736_2446365_2_1">Proceeds from issuance of common stock in initial public stock offering, net</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromIssuanceInitialPublicOffering_lbl" xml:lang="en-US" id="id_11438736_2446365_12_1">Proceeds from public stock offering, net of issuance costs</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceInitialPublicOffering" xlink:to="us-gaap_ProceedsFromIssuanceInitialPublicOffering_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xml:lang="en-US" id="id_11438736_2446016_1_1">Proceeds from Issuance of Common Stock</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xml:lang="en-US" id="id_11438736_2446016_2_1">Aggregate net proceeds received from the offering, net of underwriting discounts and commissions and estimated offering expenses payable</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl" xml:lang="en-US" id="id_11438736_2446016_12_1">Proceeds from issuance of common stock in follow on public stock offering, net</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:to="us-gaap_ProceedsFromIssuanceOfCommonStock_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet" xlink:label="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet_lbl" xml:lang="en-US" id="id_11438736_2897344_3_1">Proceeds from issuance of common stock in at-the-market stock offering, net.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet_lbl" xml:lang="en-US" id="id_11438736_2897344_1_1">Proceeds from issuance of common stock in at-the-market stock offering, net</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet_lbl" xml:lang="en-US" id="id_11438736_2897344_2_1">Proceeds from issuance of common stock in at-the-market stock offering, net</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet" xlink:to="sndx_ProceedsFromIssuanceOfCommonStockInAtMarketStockOfferingNet_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock" xlink:label="us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock_lbl" xml:lang="en-US" id="id_11438736_2446553_1_1">Proceeds from Issuance of Convertible Preferred Stock</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock_lbl" xml:lang="en-US" id="id_11438736_2446553_2_1">Proceeds received from preferred stock issuance</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock" xlink:to="us-gaap_ProceedsFromIssuanceOfConvertiblePreferredStock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromPaymentsForOtherFinancingActivities" xlink:label="us-gaap_ProceedsFromPaymentsForOtherFinancingActivities"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_lbl" xml:lang="en-US" id="id_11438736_2446210_1_1">Proceeds from (Payments for) Other Financing Activities</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_lbl" xml:lang="en-US" id="id_11438736_2446210_2_1">Other</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromPaymentsForOtherFinancingActivities" xlink:to="us-gaap_ProceedsFromPaymentsForOtherFinancingActivities_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xml:lang="en-US" id="id_11438736_2446052_1_1">Proceeds from Sale and Maturity of Available-for-sale Securities</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl" xml:lang="en-US" id="id_11438736_2446052_2_1">Proceeds from sales and maturities of short-term investments</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities" xlink:to="us-gaap_ProceedsFromSaleAndMaturityOfAvailableForSaleSecurities_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromStockOptionsExercised" xlink:label="us-gaap_ProceedsFromStockOptionsExercised"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2446426_1_1">Proceeds from Stock Options Exercised</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2446426_2_1">Proceeds from stock option exercises</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ProceedsFromStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2446426_12_1">Stock options exercised</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromStockOptionsExercised" xlink:to="us-gaap_ProceedsFromStockOptionsExercised_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ProceedsFromStockPlans" xlink:label="us-gaap_ProceedsFromStockPlans"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ProceedsFromStockPlans_lbl" xml:lang="en-US" id="id_11438736_2446427_1_1">Proceeds from Stock Plans</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ProceedsFromStockPlans_lbl" xml:lang="en-US" id="id_11438736_2446427_2_1">Proceeds from Employee Stock Purchase Plan</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ProceedsFromStockPlans" xlink:to="us-gaap_ProceedsFromStockPlans_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="us-gaap_PropertyPlantAndEquipmentNet"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xml:lang="en-US" id="id_11438736_2446103_1_1">Property, Plant and Equipment, Net</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_PropertyPlantAndEquipmentNet_lbl" xml:lang="en-US" id="id_11438736_2446103_2_1">Property and equipment, net</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_PropertyPlantAndEquipmentNet" xlink:to="us-gaap_PropertyPlantAndEquipmentNet_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeAxis" xlink:label="us-gaap_RangeAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RangeAxis_lbl" xml:lang="en-US" id="id_11438736_2447194_1_1">Range [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RangeAxis_lbl" xml:lang="en-US" id="id_11438736_2447194_2_1">Range [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RangeAxis" xlink:to="us-gaap_RangeAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RangeMember" xlink:label="us-gaap_RangeMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RangeMember_lbl" xml:lang="en-US" id="id_11438736_2446806_1_1">Range [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RangeMember_lbl" xml:lang="en-US" id="id_11438736_2446806_2_1">Range [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RangeMember" xlink:to="us-gaap_RangeMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_RecentAccountingStandardsExpectedDateOfAdoption" xlink:label="sndx_RecentAccountingStandardsExpectedDateOfAdoption"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_RecentAccountingStandardsExpectedDateOfAdoption_lbl" xml:lang="en-US" id="id_11438736_2620049_3_1">Recent accounting standards expected date of adoption.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_RecentAccountingStandardsExpectedDateOfAdoption_lbl" xml:lang="en-US" id="id_11438736_2620049_1_1">Recent Accounting Standards Expected Date of Adoption</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_RecentAccountingStandardsExpectedDateOfAdoption_lbl" xml:lang="en-US" id="id_11438736_2620049_2_1">Date of adoption of ASU 2014-09</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_RecentAccountingStandardsExpectedDateOfAdoption" xlink:to="sndx_RecentAccountingStandardsExpectedDateOfAdoption_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital" xlink:label="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital_lbl" xml:lang="en-US" id="id_11438736_2538614_3_1">Reclassification of common stock warrant liability to additional paid in capital.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital_lbl" xml:lang="en-US" id="id_11438736_2538614_1_1">Reclassification Of Common Stock Warrant Liability To Additional Paid In Capital</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital_lbl" xml:lang="en-US" id="id_11438736_2538614_2_1">Reclassification of common stock warrant liability to additional paid-in capital</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital_lbl" xml:lang="en-US" id="id_11438736_2538614_14_1">Reclassification</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital" xlink:to="sndx_ReclassificationOfCommonStockWarrantLiabilityToAdditionalPaidInCapital_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RecognitionOfDeferredRevenue" xlink:label="us-gaap_RecognitionOfDeferredRevenue"/>
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    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RepurchaseAgreementCounterpartyNameDomain" xlink:to="us-gaap_RepurchaseAgreementCounterpartyNameDomain_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2447511_1_1">Research and Development Expense</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2447511_2_1">Research and development</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpense_lbl" xml:lang="en-US" id="id_11438736_2447511_12_1">Research and development expense</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ResearchAndDevelopmentExpense" xlink:to="us-gaap_ResearchAndDevelopmentExpense_lbl"/>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xml:lang="en-US" id="id_11438736_2447512_1_1">Research and Development Expense [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpenseMember_lbl" xml:lang="en-US" id="id_11438736_2447512_2_1">Research and Development Expense [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ResearchAndDevelopmentExpenseMember" xlink:to="us-gaap_ResearchAndDevelopmentExpenseMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:label="us-gaap_ResearchAndDevelopmentExpensePolicy"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ResearchAndDevelopmentExpensePolicy_lbl" xml:lang="en-US" id="id_11438736_2447513_1_1">Research and Development Expense, Policy [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ResearchAndDevelopmentExpensePolicy_lbl" xml:lang="en-US" id="id_11438736_2447513_2_1">Research and Development</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ResearchAndDevelopmentExpensePolicy" xlink:to="us-gaap_ResearchAndDevelopmentExpensePolicy_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_lbl" xml:lang="en-US" id="id_11438736_2447522_1_1">Restricted Cash and Cash Equivalents, Current</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_lbl" xml:lang="en-US" id="id_11438736_2447522_2_1">Restricted cash</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_lbl" xml:lang="en-US" id="id_11438736_2447622_1_1">Cash and Cash Equivalents [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_lbl" xml:lang="en-US" id="id_11438736_2447622_2_1">Cash and Cash Equivalents [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember" xlink:to="us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RestrictedStockMember" xlink:label="us-gaap_RestrictedStockMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RestrictedStockMember_lbl" xml:lang="en-US" id="id_11438736_2447533_1_1">Restricted Stock [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RestrictedStockMember_lbl" xml:lang="en-US" id="id_11438736_2447533_2_1">Restricted Stock Subject to Future Vesting [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RestrictedStockMember" xlink:to="us-gaap_RestrictedStockMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xml:lang="en-US" id="id_11438736_2448061_1_1">Retained Earnings (Accumulated Deficit)</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RetainedEarningsAccumulatedDeficit_lbl" xml:lang="en-US" id="id_11438736_2448061_2_1">Accumulated deficit</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RetainedEarningsMember_lbl" xml:lang="en-US" id="id_11438736_2447681_1_1">Retained Earnings [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RetainedEarningsMember_lbl" xml:lang="en-US" id="id_11438736_2447681_2_1">Accumulated Deficit [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RetainedEarningsMember" xlink:to="us-gaap_RetainedEarningsMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevenueRecognitionMilestoneMethodFactors" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodFactors"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodFactors_lbl" xml:lang="en-US" id="id_11438736_2447601_1_1">Revenue Recognition, Milestone Method, Factors</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodFactors_lbl" xml:lang="en-US" id="id_11438736_2447601_2_1">Milestone payment agreement terms</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized_lbl" xml:lang="en-US" id="id_11438736_2447724_1_1">Revenue Recognition, Milestone Method, Revenue Recognized</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized_lbl" xml:lang="en-US" id="id_11438736_2447724_2_1">Milestone payment received</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized" xlink:to="us-gaap_RevenueRecognitionMilestoneMethodRevenueRecognized_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockNameOfTransactionDomain" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xml:lang="en-US" id="id_11438736_2447774_1_1">Sale of Stock [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SaleOfStockNameOfTransactionDomain_lbl" xml:lang="en-US" id="id_11438736_2447774_2_1">Sale of Stock [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SaleOfStockNameOfTransactionDomain" xlink:to="us-gaap_SaleOfStockNameOfTransactionDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SaleOfStockPricePerShare" xlink:label="us-gaap_SaleOfStockPricePerShare"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SaleOfStockPricePerShare_lbl" xml:lang="en-US" id="id_11438736_2448141_1_1">Sale of Stock, Price Per Share</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SaleOfStockPricePerShare_lbl" xml:lang="en-US" id="id_11438736_2448141_2_1">Sale of stock, price per share</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SaleOfStockPricePerShare" xlink:to="us-gaap_SaleOfStockPricePerShare_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="us-gaap_SalesRevenueNet"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SalesRevenueNet_lbl" xml:lang="en-US" id="id_11438736_2448151_1_1">Revenue, Net</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SalesRevenueNet_lbl" xml:lang="en-US" id="id_11438736_2448151_2_1">Total revenues</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SalesRevenueNetAbstract" xlink:label="us-gaap_SalesRevenueNetAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SalesRevenueNetAbstract_lbl" xml:lang="en-US" id="id_11438736_2447784_1_1">Revenue, Net [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SalesRevenueNetAbstract_lbl" xml:lang="en-US" id="id_11438736_2447784_2_1">Revenue:</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl" xml:lang="en-US" id="id_11438736_2448809_2_1">Weighted average grant date fair value of options</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:to="us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xml:lang="en-US" id="id_11438736_2504593_3_1">Share based compensation arrangement by share based payment award stock option grant date fair value.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xml:lang="en-US" id="id_11438736_2504593_1_1">Share Based Compensation Arrangement By Share Based Payment Award Stock Option Grant Date Fair Value</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl" xml:lang="en-US" id="id_11438736_2504593_2_1">Options grant date fair value amount</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue" xlink:to="sndx_ShareBasedCompensationArrangementByShareBasedPaymentAwardStockOptionGrantDateFairValue_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xml:lang="en-US" id="id_11438736_2448698_1_1">Equity Award [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl" xml:lang="en-US" id="id_11438736_2448698_2_1">Equity Award [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:to="us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharePrice" xlink:label="us-gaap_SharePrice"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharePrice_lbl" xml:lang="en-US" id="id_11438736_2449096_1_1">Share Price</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharePrice_lbl" xml:lang="en-US" id="id_11438736_2449096_2_1">Preferred share price per share</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharePrice" xlink:to="us-gaap_SharePrice_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharesIssuedPricePerShare" xlink:label="us-gaap_SharesIssuedPricePerShare"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharesIssuedPricePerShare_lbl" xml:lang="en-US" id="id_11438736_2449098_1_1">Shares Issued, Price Per Share</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SharesIssuedPricePerShare_lbl" xml:lang="en-US" id="id_11438736_2449098_2_1">Sale of stock, price per share</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharesIssuedPricePerShare" xlink:to="us-gaap_SharesIssuedPricePerShare_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SharesOutstanding" xlink:label="us-gaap_SharesOutstanding"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2449099_1_1">Shares, Outstanding</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_SharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2449099_4_1">Ending balance, Shares</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_SharesOutstanding_lbl" xml:lang="en-US" id="id_11438736_2449099_5_1">Beginning balance, Shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SharesOutstanding" xlink:to="us-gaap_SharesOutstanding_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_ShortTermInvestments" xlink:label="us-gaap_ShortTermInvestments"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_ShortTermInvestments_lbl" xml:lang="en-US" id="id_11438736_2449260_1_1">Short-term Investments</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_ShortTermInvestments_lbl" xml:lang="en-US" id="id_11438736_2449260_2_1">Short-term investments</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_ShortTermInvestments" xlink:to="us-gaap_ShortTermInvestments_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_SignificantAccountingPoliciesPolicyTextBlock" xlink:label="sndx_SignificantAccountingPoliciesPolicyTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2759422_3_1">Significant Accounting Policies Policy.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2759422_1_1">Significant Accounting Policies [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_SignificantAccountingPoliciesPolicyTextBlock_lbl" xml:lang="en-US" id="id_11438736_2759422_2_1">Significant Accounting Policies</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_SignificantAccountingPoliciesPolicyTextBlock" xlink:to="sndx_SignificantAccountingPoliciesPolicyTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SignificantAccountingPoliciesTextBlock" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449415_1_1">Significant Accounting Policies [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SignificantAccountingPoliciesTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449415_2_1">Summary of Significant Accounting Policies</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SignificantAccountingPoliciesTextBlock" xlink:to="us-gaap_SignificantAccountingPoliciesTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_SignificantAgreementsDisclosureTextBlock" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2770222_3_1">Significant agreements disclosure.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2770222_1_1">Significant Agreements Disclosure [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_SignificantAgreementsDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2770222_2_1">Significant Agreements</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_SignificantAgreementsDisclosureTextBlock" xlink:to="sndx_SignificantAgreementsDisclosureTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementClassOfStockAxis" xlink:label="us-gaap_StatementClassOfStockAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementClassOfStockAxis_lbl" xml:lang="en-US" id="id_11438736_2448913_1_1">Class of Stock [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StatementClassOfStockAxis_lbl" xml:lang="en-US" id="id_11438736_2448913_2_1">Class of Stock [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementClassOfStockAxis" xlink:to="us-gaap_StatementClassOfStockAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="us-gaap_StatementEquityComponentsAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xml:lang="en-US" id="id_11438736_2448914_1_1">Equity Components [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StatementEquityComponentsAxis_lbl" xml:lang="en-US" id="id_11438736_2448914_2_1">Equity Components [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementEquityComponentsAxis" xlink:to="us-gaap_StatementEquityComponentsAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StatementOfFinancialPositionAbstract" xlink:label="us-gaap_StatementOfFinancialPositionAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StatementOfFinancialPositionAbstract_lbl" xml:lang="en-US" id="id_11438736_2449303_1_1">Statement of Financial Position [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StatementOfFinancialPositionAbstract_lbl" xml:lang="en-US" id="id_11438736_2449303_2_1">Statement of Financial Position [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StatementOfFinancialPositionAbstract" xlink:to="us-gaap_StatementOfFinancialPositionAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions" xlink:label="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions_lbl" xml:lang="en-US" id="id_11438736_2521175_3_1">Stock Based Compensation Due To Accelerated Vesting Of Stock Options</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions_lbl" xml:lang="en-US" id="id_11438736_2521175_1_1">Stock Based Compensation Due To Accelerated Vesting Of Stock Options</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions_lbl" xml:lang="en-US" id="id_11438736_2521175_2_1">Additional stock compensation expense</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions" xlink:to="sndx_StockBasedCompensationDueToAcceleratedVestingOfStockOptions_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="us-gaap_StockholdersEquity"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2449320_1_1">Stockholders' Equity Attributable to Parent</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2449320_4_1">Ending balance</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2449320_5_1">Beginning balance</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/totalLabel" xlink:label="us-gaap_StockholdersEquity_lbl" xml:lang="en-US" id="id_11438736_2449320_6_1">Total stockholders' equity</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquity" xlink:to="us-gaap_StockholdersEquity_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="us-gaap_StockholdersEquityAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xml:lang="en-US" id="id_11438736_2449321_1_1">Stockholders' Equity Attributable to Parent [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockholdersEquityAbstract_lbl" xml:lang="en-US" id="id_11438736_2449321_2_1">Stockholders' equity:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquityAbstract" xlink:to="us-gaap_StockholdersEquityAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449326_1_1">Stockholders' Equity Note Disclosure [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449326_2_1">Stockholders' Equity</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockholdersEquityNoteDisclosureTextBlock" xlink:to="us-gaap_StockholdersEquityNoteDisclosureTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_lbl" xml:lang="en-US" id="id_11438736_2449334_1_1">Stock Issued During Period, Shares, Employee Stock Purchase Plans</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_lbl" xml:lang="en-US" id="id_11438736_2449334_2_1">Employee withholdings ESPP, Shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:to="us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" id="id_11438736_2449858_1_1">Stock Issued During Period, Shares, New Issues</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" id="id_11438736_2449858_2_1">Common stock, new shares issued</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl" xml:lang="en-US" id="id_11438736_2449858_12_1">Proceeds from follow on offering, Shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodSharesNewIssues" xlink:to="us-gaap_StockIssuedDuringPeriodSharesNewIssues_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram" xlink:label="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2608732_3_1">Stock issued during period shares new issues under the at the market program.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2608732_1_1">Stock Issued During Period Shares New Issues Under The At The Market Program</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2608732_2_1">Proceeds from "At-the-market" offering, net, shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram" xlink:to="sndx_StockIssuedDuringPeriodSharesNewIssuesUnderTheAtTheMarketProgram_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" xlink:label="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_lbl" xml:lang="en-US" id="id_11438736_2449474_1_1">Stock Issued During Period, Shares, Restricted Stock Award, Gross</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_lbl" xml:lang="en-US" id="id_11438736_2449474_2_1">Vesting of restricted stock, Shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross" xlink:to="us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2449339_1_1">Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2449339_2_1">Proceeds from exercise of stock options, Shares</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/verboseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2449339_12_1">Stock options exercised, shares</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:to="us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_lbl" xml:lang="en-US" id="id_11438736_2449343_1_1">Stock Issued During Period, Value, Employee Stock Purchase Plan</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_lbl" xml:lang="en-US" id="id_11438736_2449343_2_1">Employee withholdings ESPP</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan" xlink:to="us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xml:lang="en-US" id="id_11438736_2449480_1_1">Stock Issued During Period, Value, New Issues</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl" xml:lang="en-US" id="id_11438736_2449480_2_1">Proceeds from follow-on offering, Value</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodValueNewIssues" xlink:to="us-gaap_StockIssuedDuringPeriodValueNewIssues_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram" xlink:label="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2538887_3_1">Stock issued during period value new issues under the at the market program.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2538887_1_1">Stock Issued During Period Value New Issues Under The At The Market Program</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram_lbl" xml:lang="en-US" id="id_11438736_2538887_2_1">Proceeds from "At-the-market" offering, net</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram" xlink:to="sndx_StockIssuedDuringPeriodValueNewIssuesUnderTheAtTheMarketProgram_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross_lbl" xml:lang="en-US" id="id_11438736_2449594_1_1">Stock Issued During Period, Value, Restricted Stock Award, Gross</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross_lbl" xml:lang="en-US" id="id_11438736_2449594_2_1">Vesting of restricted stock, Value</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross" xlink:to="us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardGross_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2449485_1_1">Stock Issued During Period, Value, Stock Options Exercised</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_lbl" xml:lang="en-US" id="id_11438736_2449485_2_1">Proceeds from exercise of stock options, Value</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised" xlink:to="us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="us-gaap_SubsequentEventMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsequentEventMember_lbl" xml:lang="en-US" id="id_11438736_2449609_1_1">Subsequent Event [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsequentEventMember_lbl" xml:lang="en-US" id="id_11438736_2449609_2_1">Subsequent Event [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsequentEventMember" xlink:to="us-gaap_SubsequentEventMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventsAbstract" xlink:label="us-gaap_SubsequentEventsAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsequentEventsAbstract_lbl" xml:lang="en-US" id="id_11438736_2449610_1_1">Subsequent Events [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsequentEventsAbstract_lbl" xml:lang="en-US" id="id_11438736_2449610_2_1">Subsequent Events [Abstract]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsequentEventsAbstract" xlink:to="us-gaap_SubsequentEventsAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventsTextBlock" xlink:label="us-gaap_SubsequentEventsTextBlock"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsequentEventsTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449887_1_1">Subsequent Events [Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsequentEventsTextBlock_lbl" xml:lang="en-US" id="id_11438736_2449887_2_1">Subsequent Event</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsequentEventsTextBlock" xlink:to="us-gaap_SubsequentEventsTextBlock_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="us-gaap_SubsequentEventTypeAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsequentEventTypeAxis_lbl" xml:lang="en-US" id="id_11438736_2449888_1_1">Subsequent Event Type [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsequentEventTypeAxis_lbl" xml:lang="en-US" id="id_11438736_2449888_2_1">Subsequent Event Type [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsequentEventTypeAxis" xlink:to="us-gaap_SubsequentEventTypeAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="us-gaap_SubsequentEventTypeDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsequentEventTypeDomain_lbl" xml:lang="en-US" id="id_11438736_2449611_1_1">Subsequent Event Type [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsequentEventTypeDomain_lbl" xml:lang="en-US" id="id_11438736_2449611_2_1">Subsequent Event Type [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsequentEventTypeDomain" xlink:to="us-gaap_SubsequentEventTypeDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SubsidiarySaleOfStockAxis" xlink:label="us-gaap_SubsidiarySaleOfStockAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xml:lang="en-US" id="id_11438736_2449620_1_1">Sale of Stock [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SubsidiarySaleOfStockAxis_lbl" xml:lang="en-US" id="id_11438736_2449620_2_1">Sale of Stock [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SubsidiarySaleOfStockAxis" xlink:to="us-gaap_SubsidiarySaleOfStockAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_SuccessFeePayableMember" xlink:label="sndx_SuccessFeePayableMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_SuccessFeePayableMember_lbl" xml:lang="en-US" id="id_11438736_2695971_3_1">Success fee payable.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_SuccessFeePayableMember_lbl" xml:lang="en-US" id="id_11438736_2695971_1_1">Success Fee Payable [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_SuccessFeePayableMember_lbl" xml:lang="en-US" id="id_11438736_2695971_2_1">Success Fee Payable [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_SuccessFeePayableMember" xlink:to="sndx_SuccessFeePayableMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_SummaryOfSignificantAccountingPoliciesLineItems" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesLineItems"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesLineItems_lbl" xml:lang="en-US" id="id_11438736_2460318_3_1">Summary of Significant Accounting Policies [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesLineItems_lbl" xml:lang="en-US" id="id_11438736_2460318_1_1">Summary Of Significant Accounting Policies [Line Items]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesLineItems_lbl" xml:lang="en-US" id="id_11438736_2460318_2_1">Summary Of Significant Accounting Policies [Line Items]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_SummaryOfSignificantAccountingPoliciesLineItems" xlink:to="sndx_SummaryOfSignificantAccountingPoliciesLineItems_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_SummaryOfSignificantAccountingPoliciesTable" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesTable"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesTable_lbl" xml:lang="en-US" id="id_11438736_2648468_3_1">Summary Of Significant Accounting Policies [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesTable_lbl" xml:lang="en-US" id="id_11438736_2648468_1_1">Summary Of Significant Accounting Policies [Table]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_SummaryOfSignificantAccountingPoliciesTable_lbl" xml:lang="en-US" id="id_11438736_2648468_2_1">Summary Of Significant Accounting Policies [Table]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_SummaryOfSignificantAccountingPoliciesTable" xlink:to="sndx_SummaryOfSignificantAccountingPoliciesTable_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract_lbl" xml:lang="en-US" id="id_11438736_2450036_1_1">Supplemental Cash Flow Information [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_SupplementalCashFlowInformationAbstract_lbl" xml:lang="en-US" id="id_11438736_2450036_2_1">SUPPLEMENTAL DISCLOSURE OF CASH FLOW INFORMATION:</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_SupplementalCashFlowInformationAbstract" xlink:to="us-gaap_SupplementalCashFlowInformationAbstract_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_Supplies" xlink:label="us-gaap_Supplies"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_Supplies_lbl" xml:lang="en-US" id="id_11438736_2450044_1_1">Prepaid Supplies</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_Supplies_lbl" xml:lang="en-US" id="id_11438736_2450044_2_1">Prepaid clinical supplies</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_Supplies" xlink:to="us-gaap_Supplies_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TemporaryEquityDividendsAdjustment" xlink:label="us-gaap_TemporaryEquityDividendsAdjustment"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_TemporaryEquityDividendsAdjustment_lbl" xml:lang="en-US" id="id_11438736_2449572_1_1">Temporary Equity, Dividends, Adjustment</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_TemporaryEquityDividendsAdjustment_lbl" xml:lang="en-US" id="id_11438736_2449572_2_1">Accretion of dividends on convertible preferred stock</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2009/role/negatedLabel" xlink:label="us-gaap_TemporaryEquityDividendsAdjustment_lbl" xml:lang="en-US" id="id_11438736_2449572_14_1">Accretion of convertible preferred stock dividends</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_TemporaryEquityDividendsAdjustment" xlink:to="us-gaap_TemporaryEquityDividendsAdjustment_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_TermLoanMember" xlink:label="sndx_TermLoanMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_TermLoanMember_lbl" xml:lang="en-US" id="id_11438736_2727967_3_1">Term Loan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_TermLoanMember_lbl" xml:lang="en-US" id="id_11438736_2727967_1_1">Term Loan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_TermLoanMember_lbl" xml:lang="en-US" id="id_11438736_2727967_2_1">Term Loan [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_TermLoanMember" xlink:to="sndx_TermLoanMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_TradingSymbol" xlink:label="dei_TradingSymbol"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="dei_TradingSymbol_lbl" xml:lang="en-US" id="id_11438736_2450288_1_1">Trading Symbol</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="dei_TradingSymbol_lbl" xml:lang="en-US" id="id_11438736_2450288_2_1">Trading Symbol</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="dei_TradingSymbol" xlink:to="dei_TradingSymbol_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_lbl" xml:lang="en-US" id="id_11438736_2449742_1_1">Financial Instruments [Domain]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_lbl" xml:lang="en-US" id="id_11438736_2449742_2_1">Financial Instruments [Domain]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:to="us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_TwoThousandAndSevenStockIncentivePlanMember" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2668893_3_1">Two Thousand And Seven Stock Incentive Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2668893_1_1">Two Thousand And Seven Stock Incentive Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2668893_2_1">2007 Plan [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_TwoThousandAndSevenStockIncentivePlanMember" xlink:to="sndx_TwoThousandAndSevenStockIncentivePlanMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember" xlink:label="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember_lbl" xml:lang="en-US" id="id_11438736_2661741_3_1">2015 Employee Stock Purchase Plan.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember_lbl" xml:lang="en-US" id="id_11438736_2661741_1_1">Two Thousand Fifteen Employee Stock Purchase Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember_lbl" xml:lang="en-US" id="id_11438736_2661741_2_1">2015 ESPP [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember" xlink:to="sndx_TwoThousandFifteenEmployeeStockPurchasePlanMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_TwoThousandFifteenOmnibusIncentivePlanMember" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2723146_3_1">Two Thousand Fifteen Omnibus Incentive Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2723146_1_1">Two Thousand Fifteen Omnibus Incentive Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2723146_2_1">2015 Plan [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_TwoThousandFifteenOmnibusIncentivePlanMember" xlink:to="sndx_TwoThousandFifteenOmnibusIncentivePlanMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2699876_3_1">Two thousand seven and two thousand fifteen stock incentive plan.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2699876_1_1">Two Thousand Seven And Two Thousand Fifteen Stock Incentive Plan [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl" xml:lang="en-US" id="id_11438736_2699876_2_1">2015 and 2007 [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember" xlink:to="sndx_TwoThousandSevenAndTwoThousandFifteenStockIncentivePlanMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_TypeOfArrangementAxis" xlink:label="us-gaap_TypeOfArrangementAxis"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xml:lang="en-US" id="id_11438736_2450320_1_1">Type of Arrangement and Non-arrangement Transactions [Axis]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_TypeOfArrangementAxis_lbl" xml:lang="en-US" id="id_11438736_2450320_2_1">Type of Arrangement and Non-arrangement Transactions [Axis]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_TypeOfArrangementAxis" xlink:to="us-gaap_TypeOfArrangementAxis_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UCBBiopharmaMember" xlink:label="sndx_UCBBiopharmaMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UCBBiopharmaMember_lbl" xml:lang="en-US" id="id_11438736_2693826_3_1">UCB Biopharma.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UCBBiopharmaMember_lbl" xml:lang="en-US" id="id_11438736_2693826_1_1">UCB Biopharma [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UCBBiopharmaMember_lbl" xml:lang="en-US" id="id_11438736_2693826_2_1">UCB Biopharma [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UCBBiopharmaMember" xlink:to="sndx_UCBBiopharmaMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UCBLicenseAgreementMember" xlink:label="sndx_UCBLicenseAgreementMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UCBLicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2693825_3_1">UCB license agreement.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UCBLicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2693825_1_1">UCB License Agreement [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UCBLicenseAgreementMember_lbl" xml:lang="en-US" id="id_11438736_2693825_2_1">UCB License Agreement [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UCBLicenseAgreementMember" xlink:to="sndx_UCBLicenseAgreementMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UnderwritingDiscountsAndCommissions" xlink:label="sndx_UnderwritingDiscountsAndCommissions"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UnderwritingDiscountsAndCommissions_lbl" xml:lang="en-US" id="id_11438736_2489680_3_1">Underwriting discounts and commissions.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UnderwritingDiscountsAndCommissions_lbl" xml:lang="en-US" id="id_11438736_2489680_1_1">Underwriting Discounts And Commissions</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UnderwritingDiscountsAndCommissions_lbl" xml:lang="en-US" id="id_11438736_2489680_2_1">Underwriting discounts and commissions</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UnderwritingDiscountsAndCommissions" xlink:to="sndx_UnderwritingDiscountsAndCommissions_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UpfrontLicenseFeesPaid" xlink:label="sndx_UpfrontLicenseFeesPaid"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xml:lang="en-US" id="id_11438736_2577306_3_1">Upfront license fees paid.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xml:lang="en-US" id="id_11438736_2577306_1_1">Upfront License Fees Paid</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UpfrontLicenseFeesPaid_lbl" xml:lang="en-US" id="id_11438736_2577306_2_1">Up-front license fee paid</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UpfrontLicenseFeesPaid" xlink:to="sndx_UpfrontLicenseFeesPaid_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UpfrontPaymentAllocatedToLicenseFee" xlink:label="sndx_UpfrontPaymentAllocatedToLicenseFee"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UpfrontPaymentAllocatedToLicenseFee_lbl" xml:lang="en-US" id="id_11438736_2518726_3_1">Upfront payment allocated to license fee.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UpfrontPaymentAllocatedToLicenseFee_lbl" xml:lang="en-US" id="id_11438736_2518726_1_1">Upfront Payment Allocated to License Fee</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UpfrontPaymentAllocatedToLicenseFee_lbl" xml:lang="en-US" id="id_11438736_2518726_2_1">Up-front payment allocated to license fee</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UpfrontPaymentAllocatedToLicenseFee" xlink:to="sndx_UpfrontPaymentAllocatedToLicenseFee_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UpfrontPayments" xlink:label="sndx_UpfrontPayments"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UpfrontPayments_lbl" xml:lang="en-US" id="id_11438736_2559063_3_1">Upfront payments</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UpfrontPayments_lbl" xml:lang="en-US" id="id_11438736_2559063_1_1">Upfront Payments</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UpfrontPayments_lbl" xml:lang="en-US" id="id_11438736_2559063_2_1">Upfront milestone payable</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UpfrontPayments" xlink:to="sndx_UpfrontPayments_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_UpfrontPaymentsReceived" xlink:label="sndx_UpfrontPaymentsReceived"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xml:lang="en-US" id="id_11438736_2507396_3_1">Upfront payments received</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xml:lang="en-US" id="id_11438736_2507396_1_1">Upfront payments received</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_UpfrontPaymentsReceived_lbl" xml:lang="en-US" id="id_11438736_2507396_2_1">License agreement upfront payment received</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_UpfrontPaymentsReceived" xlink:to="sndx_UpfrontPaymentsReceived_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_UseOfEstimates" xlink:label="us-gaap_UseOfEstimates"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_UseOfEstimates_lbl" xml:lang="en-US" id="id_11438736_2450913_1_1">Use of Estimates, Policy [Policy Text Block]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_UseOfEstimates_lbl" xml:lang="en-US" id="id_11438736_2450913_2_1">Use of Estimates</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_UseOfEstimates" xlink:to="us-gaap_UseOfEstimates_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_VestingOfRestrictedStockValue" xlink:label="sndx_VestingOfRestrictedStockValue"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_VestingOfRestrictedStockValue_lbl" xml:lang="en-US" id="id_11438736_2576189_3_1">Vesting of restricted stock value.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_VestingOfRestrictedStockValue_lbl" xml:lang="en-US" id="id_11438736_2576189_1_1">Vesting Of Restricted Stock Value</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_VestingOfRestrictedStockValue_lbl" xml:lang="en-US" id="id_11438736_2576189_2_1">Vesting of restricted stock</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_VestingOfRestrictedStockValue" xlink:to="sndx_VestingOfRestrictedStockValue_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_VitaePharmaceuticalsIncMember" xlink:label="sndx_VitaePharmaceuticalsIncMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xml:lang="en-US" id="id_11438736_2897354_3_1">Vitae Pharmaceuticals, Inc.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xml:lang="en-US" id="id_11438736_2897354_1_1">Vitae Pharmaceuticals, Inc. [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_VitaePharmaceuticalsIncMember_lbl" xml:lang="en-US" id="id_11438736_2897354_2_1">Vitae Pharmaceuticals, Inc. [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_VitaePharmaceuticalsIncMember" xlink:to="sndx_VitaePharmaceuticalsIncMember_lbl"/>
    <loc xlink:type="locator" xlink:href="sndx-20170930.xsd#sndx_WarrantExpirationTerm" xlink:label="sndx_WarrantExpirationTerm"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/documentation" xlink:label="sndx_WarrantExpirationTerm_lbl" xml:lang="en-US" id="id_11438736_2632184_3_1">Warrant expiration term.</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="sndx_WarrantExpirationTerm_lbl" xml:lang="en-US" id="id_11438736_2632184_1_1">Warrant Expiration Term</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="sndx_WarrantExpirationTerm_lbl" xml:lang="en-US" id="id_11438736_2632184_2_1">Warrant expiration period</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="sndx_WarrantExpirationTerm" xlink:to="sndx_WarrantExpirationTerm_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_WarrantMember" xlink:label="us-gaap_WarrantMember"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WarrantMember_lbl" xml:lang="en-US" id="id_11438736_2450820_1_1">Warrant [Member]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_WarrantMember_lbl" xml:lang="en-US" id="id_11438736_2450820_2_1">Common Stock Warrant [Member]</label>
    <labelArc xlink:type="arc" xlink:arcrole="http://www.xbrl.org/2003/arcrole/concept-label" xlink:from="us-gaap_WarrantMember" xlink:to="us-gaap_WarrantMember_lbl"/>
    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_WarrantsAndRightsOutstanding" xlink:label="us-gaap_WarrantsAndRightsOutstanding"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WarrantsAndRightsOutstanding_lbl" xml:lang="en-US" id="id_11438736_2450549_1_1">Warrants and Rights Outstanding</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodEndLabel" xlink:label="us-gaap_WarrantsAndRightsOutstanding_lbl" xml:lang="en-US" id="id_11438736_2450549_4_1">Ending Balance</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/periodStartLabel" xlink:label="us-gaap_WarrantsAndRightsOutstanding_lbl" xml:lang="en-US" id="id_11438736_2450549_5_1">Beginning Balance</label>
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    <loc xlink:type="locator" xlink:href="http://xbrl.fasb.org/us-gaap/2017/elts/us-gaap-2017-01-31.xsd#us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted" xlink:label="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted"/>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WeightedAverageNumberOfShareOutstandingBasicAndDiluted_lbl" xml:lang="en-US" id="id_11438736_2450563_1_1">Weighted Average Number of Shares Outstanding, Basic and Diluted</label>
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    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/label" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_lbl" xml:lang="en-US" id="id_11438736_2450829_1_1">Weighted Average Number of Shares Outstanding, Diluted [Abstract]</label>
    <label xlink:type="resource" xlink:role="http://www.xbrl.org/2003/role/terseLabel" xlink:label="us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_lbl" xml:lang="en-US" id="id_11438736_2450829_2_1">Denominator-basic and diluted:</label>
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