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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 Delaware State or other jurisdiction of incorporation or organization SPERO THERAPEUTICS , INC . ( Exact name of registrant as specified in its charter ) 675 Massachusetts Avenue , 14th Floor Cambridge , Massachusetts ( Address of principal executive offices ) Commission file number 001-38266 Title of each class Common Stock , $ 0.001 par value per share Large accelerated filer Non - accelerated filer ( Zip Code ) Registrant's telephone number , including area code ( 857 ) 242-1600 X 46-4590683 ( I.R.S. Employer Identification No. ) 02139 Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol ( s ) SPRO Name of each exchange on which registered The Nasdaq Global Select Market Indicate by check mark if the registrant is a well - known seasoned issuer , as defined Rule 405 of the Securities Act . Yes No X Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or 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 Securities registered pursuant to Section 12 ( g ) of the Act : None 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 . Accelerated filer Smaller reporting company Emerging growth company 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 Act ) . Yes □ No | | 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 . X DOCUMENTS INCORPORATED BY REFERENCE The registrant intends to file a definitive proxy statement pursuant to Regulation 14A relating to the 2021 Annual Meeting of Stockholders within 120 days of the end of the registrant's fiscal year ended December 31 , 2020. Portions of such definitive proxy statement are incorporated by reference into Part III of this Annual Report on Form 10 - K to the extent stated herein . 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 $ 256.2 million ( based on the last reported sale price on the Nasdaq Global Market as of such date ) . As of March 8 , 2021 , there were 29,504,257 shares of the registrant's Common Stock , $ 0.001 par value per share , outstanding .