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Table of Contents UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 Form 10 - K ANNUAL REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the year ended December 31 , 2020 OR TRANSITION REPORT PURSUANT TO SECTION 13 OR 15 ( d ) OF THE SECURITIES EXCHANGE ACT OF 1934 For the period from to Commission file number 0-17999 ImmunoGen , Inc. Massachusetts ( State or other jurisdiction of incorporation or organization ) 04-2726691 ( I.R.S. Employer Identification No. ) 830 Winter Street , Waltham , MA 02451 ( Address of principal executive offices , including zip code ) ( 781 ) 895-0600 Title of Each Class ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) Name of Each Exchange on Which Registered Common Stock , $ .01 par value IMGN Nasdaq Global Select Market 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 Section 15 ( d ) of the 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 such 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 ( §229.405 of this chapter ) during the preceding 12 months ( or for such shorter period that the registrant was required to submit such files ) . Yes No Large accelerated filer > Non - accelerated filer 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 definitions of " large accelerated filer , " " accelerated filer , " " smaller reporting company , " and " emerging growth company " in Rule 12b - 2 of the Exchange Act .: Accelerated filer Smaller reporting company 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 financial accounting standards provided pursuant to Section 13 ( a ) of the Exchange Act . □ 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 in Rule 12b - 2 of the Exchange Act ) . Yes No Aggregate market value , based upon the closing sale price of the shares as reported by the Nasdaq Global Select Market , of common stock held by non - affiliates at June 30 , 2020 : $ 799,032,429 ( excludes shares held by executive officers and directors ) . Exclusion of shares held by any person should not be construed to indicate that such person possesses the power , direct or indirect , to direct or cause the direction of management or policies of the registrant , or that such person is controlled by or under common control with the registrant . Common stock outstanding at February 18 , 2021 : 199,731,834 shares . DOCUMENTS INCORPORATED BY REFERENCE Portions of the definitive Proxy Statement on Schedule 14A to be delivered to shareholders in connection with the Annual Meeting of Shareholders to be held on June 16 , 2021 are incorporated by reference into Part III of this report .