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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 ΤΟ Commission File Number 001-37566 Delaware ( State or other jurisdiction of incorporation or organization ) SYNLOGIC , INC . ( Exact name of Registrant as specified in its Charter ) 301 Binney St. , Suite 402 Cambridge , MA ( Address of principal executive offices ) Title of each class Common Stock , par value $ 0.001 per share Large accelerated filer Non - accelerated filer ( 617 ) 401-9975 ( Registrant's telephone number , including area code ) Securities registered pursuant to Section 12 ( b ) of the Act : Trading Symbol SYBX □□□ Securities registered pursuant to Section 12 ( g ) of The Act : None 26-1824804 ( I.R.S. Employer Identification No. ) 02142 ( Zip Code ) 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 Name of exchange on which registered The Nasdaq Capital Market 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 , 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 Small 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 . □ 风口 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 as of June 30 , 2020 , the last business day of the registrant's most recently completed second quarter , was $ 38.0 million , computed based on the closing price of $ 2.03 per share on June 30 , 2020 . As of March 18 , 2021 there were 39,657,841 shares of the registrant's common stock , par value $ 0.001 per share , outstanding . DOCUMENTS INCORPORATED BY REFERENCE The following documents ( or parts thereof ) are incorporated by reference into the following parts of this Form 10 - K : Certain information required in Part III of this Annual Report on Form 10 - K is incorporated from the registrant's definitive proxy statement for the 2021 annual meeting of stockholders to be filed pursuant to Regulation 14A with the Securities and Exchange Commission within 120 days of the registrant's fiscal year ended December 31 , 2020 .