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Table of Contents ( Mark One ) 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 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 : 001-39319 GENERATION BIO CO . ( Exact name of registrant as specified in its charter ) 81-4301284 ( I.R.S. Employer Identification Number ) Delaware ( State or other jurisdiction of incorporation or organization ) 301 Binney Street Cambridge , Massachusetts ( Address of principal executive offices ) ( 617 ) 655-7500 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : 02142 ( Zip Code ) Title of each class Common Stock , $ 0.0001 Par Value Trading Symbol ( s ) GBIO Securities registered pursuant to Section 12 ( g ) of the Act : Name of each exchange on which registered Nasdaq Global Select Market 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 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 ( $ 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 Emerging growth company П ☑ 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 ☑ The aggregate market value of Common Stock held by non - affiliates of the registrant computed by reference to the price of the registrant's Common Stock as of June 30 , 2020 , the last business day of the registrant's most recently completed second fiscal quarter , was approximately $ 646.2 million ( based on the last reported sale price on the Nasdaq Global Select Market as of such date ) . For this computation , the registrant has excluded the market value of all shares of Common Stock reported as beneficially owned by its executive officer and directors ; such exclusion shall not be deemed to constitute an admission that any such person is an affiliate of the registrant . As of February 28 , 2021 , there were 56,427,399 shares of the registrant's Common Stock , $ 0.0001 par value per share , outstanding . DOCUMENTS INCORPORATED BY REFERENCE Portions of the registrant's Proxy Statement for its 2021 Annual Meeting of Stockholders , which the registrant intends to file 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