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( 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 ΤΟ Commission File Number 001-39402 ANNEXON , INC . ( Exact name of Registrant as specified in its Charter ) Delaware ( State or other jurisdiction of incorporation or organization ) 27-5414423 ( I.R.S. Employer Identification No. ) 180 Kimball Way , Suite 200 South San Francisco , California 94080 ( Address of principal executive offices including zip code ) Registrant's telephone number , including area code : ( 650 ) 822-5500 Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Common stock , par value $ 0.001 per share Securities registered pursuant to Section 12 ( g ) of the Act : None Trading Symbol ( s ) ANNX Name of each exchange on which registered The Nasdaq Stock 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 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 ☑ ☑ 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 Registrant was not a public company as of June 30 , 2020 , the last business day of its most recently completed second fiscal quarter , and therefore , cannot calculate the aggregate market value of its voting and non - voting common equity held by non - affiliates as of such date . The number of shares of Registrant's Common Stock outstanding as of March 15 , 2021 was 38,157,618 . DOCUMENTS INCORPORATED BY REFERENCE Portions of the Registrant's Definitive Proxy Statement relating to the Annual Meeting of Stockholders , which will be filed with the Securities and Exchange Commission within 120 days after the end of the Registrant's fiscal year ended December 31 , 2020 , are incorporated by reference into Part III of this Report .