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Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 FORM 10 - K ( Mark One ) X 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 Commission file number 001-38129 Delaware ( State or Other Jurisdiction of Incorporation or Organization ) 840 Memorial Drive Cambridge , MA ( Address of Principal Executive Offices ) Title of each class Common Stock , $ 0.0001 par value Mersana Therapeutics , Inc. ( Exact name of registrant as specified in its charter ) to Large accelerated filer Non - accelerated filer ( I.R.S. Employer Identification No. ) Registrant's telephone number , including area code ( 617 ) 498-0020 Securities registered pursuant to Section 12 ( b ) of the Act : Trading symbol ( s ) MRSN Securities registered pursuant to Section 12 ( g ) of the Act : NONE 04-3562403 02139 ( Zip Code ) Yes No X Indicate by check mark if the registrant is a well - known seasoned issuer , as defined in Rule 405 of the Securities Act . Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15 ( d ) of the Act . 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 registrant was required to file such reports ) , and ( 2 ) has been subject to such filing requirements for the past 90 days . Yes No Name of each exchange on which registered The Nasdaq Global Select Market Accelerated filer Smaller reporting company Emerging growth company 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 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 . | 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 financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . 0 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 . Indicate by check mark whether the registrant is a shell company ( as defined by Rule 12b - 2 of the Exchange Act ) . Yes No X As of June 30 , 2020 , the last business day of the registrant's most recently completed second fiscal quarter , the aggregate market value of the registrant's common stock held by non - affiliates was $ 1,147,647,431 , based on the last reported sale price of such stock on the Nasdaq Global Select Market as of such date . As of February 23 , 2021 , the registrant had 69,042,942 shares of common stock outstanding at a par value $ 0.0001 per share . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's definitive proxy statement that will be filed for the 2021 Annual Meeting of Stockholders are incorporated by reference in Part III .