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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ( Mark One ) ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the Fiscal Year Ended December 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the Transition Period from to Commission File Number : 001-37708 Delaware ( State or Other Jurisdiction of Incorporation or Organization ) Syndax Pharmaceuticals , Inc. ( Exact name of Registrant as specified in its charter ) Title of each class Common Stock 2834 ( Primary Standard Industrial Classification Code Number ) 35 Gatehouse Drive , Building D , Floor 3 Waltham , Massachusetts 02451 ( 781 ) 419-1400 ( Address , Including Zip Code , and Telephone Number , Including Area Code , of Registrant's Principal Executive Offices ) Securities registered pursuant to Section 12 ( b ) of the Act : 32-0162505 ( I.R.S. Employer Identification Number ) Trading Symbol ( s ) SNDX Name of each exchange on which registered The Nasdaq Stock Market , LLC Securities registered pursuant to Section 12 ( g ) of the Act : None Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Yes No Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 15 ( d ) of the Securities Act . Yes No Indicate by check mark whether the registrant ( 1 ) has filed all reports required to be filed by Section 13 or 15 ( d ) of the Securities Exchange Act of 1934 during the preceding 12 months ( or for such shorter period that the registrant was required to file such reports ) , and ( 2 ) has been subject to the filing requirements for the past 90 days . Yes No Indicate by check mark whether the registrant has submitted electronically every Interactive Data File required to be submitted pursuant to Rule 405 of Regulation S - T ( § 232.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes No Indicate by check mark whether the registrant is a large accelerated filer , an accelerated filer , a non - accelerated filer , a smaller reporting company , or an emerging growth company . See the definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company " and " emerging growth company " in Rule 12b - 2 of the Exchange Act . Large Accelerated Filer Non - accelerated Filer Accelerated Filer Smaller Reporting Company 0 Emerging growth company If an emerging growth company , indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . Indicate by check mark whether the registrant is a shell company ( as defined in Rule 12b - 2 of the Exchange Act . ) Yes No > Indicate by check mark whether the registrant has filed a report on and attestation to its management's assessment of the effectiveness of its internal control over financial reporting under Section 404 ( b ) of the Sarbanes - Oxley Act ( 15 U.S.C. 7262 ( b ) ) by the registered public accounting firm that prepared or issued its audit report . As of June 30 , 2020 , the last day of the registrant's most recently completed second fiscal quarter , the aggregate market value of the Common Stock held by non - affiliates of the registrant was approximately $ 500.1 million , based on the closing price of the registrant's common stock on June 30 , 2020. Shares of the registrant's common stock held by each officer and director and stockholders that the registrant has concluded are affiliates of the registrant . This determination of affiliate status is not a determination for other purposes . As of March 10 , 2021 , there were 48,235,759 shares of common stock outstanding . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's definitive proxy statement for its 2021 Annual Meeting of Stockholders , which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant's fiscal year ended December 31 , 2020 , are incorporated by reference into Part III of this Annual Report on Form 10 - K .