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UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington , D.C. 20549 ( 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 FORM 10 - K Commission File Number : 001-39264 Delaware ( State or other jurisdiction of incorporation or organization ) 99 Hayden Avenue , Suite 120 , Building E Lexington , Massachusetts Non - accelerated filer KEROS THERAPEUTICS , INC . ( Exact name of registrant as specified in its charter ) ( Address of principal executive offices ) Tel : ( 617 ) 314-6297 ( Registrant's telephone number , including area code ) to Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol KROS 1 Title of each class Common Stock , $ 0.0001 par value per share 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 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 □ Accelerated filer ☐ Smaller reporting company 81-1173868 ( I.R.S. Employer Identification Number ) Emerging growth company 02421 ( Zip Code ) Name of each exchange on which registered The Nasdaq Stock Market LLC 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 .