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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 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 001-36644 For the fiscal year ended December 31 , 2020 OR Delaware ( State or other jurisdiction of incorporation or organization ) CALITHERA BIOSCIENCES , INC . ( Exact name of Registrant as specified in its Charter ) 343 Oyster Point Blvd. , Suite 200 South San Francisco , CA ( Address of principal executive offices ) Common Stock , Par Value $ 0.0001 Per Share ( Title of each class ) 27-2366329 ( I.R.S. Employer Identification No. ) Registrant's telephone number , including area code : ( 650 ) 870-1000 Securities registered pursuant to Section 12 ( b ) of the Act : CALA X 94080 ( Zip Code ) The Nasdaq Global Select Market ( Name of each exchange on which registered ) ( Trading Symbol ) 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 Non - accelerated filer 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 . Accelerated filer Smaller reporting 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 Exchange Act ) . YES □ NO > The aggregate market value of the registrant's common stock held by non - affiliates of the registrant as of the last business day of the registrant's most recently completed second fiscal quarter was approximately $ 345.0 million , based on the closing price of the registrant's common stock on the Nasdaq Global Select Market of $ 5.28 per share . The number of shares of Registrant's Common Stock outstanding as of March 10 , 2021 , was 73,317,051 . DOCUMENTS INCORPORATED BY REFERENCE The Registrant's Definitive Proxy Statement relating to the 2021 Annual Meeting of Stockholders will be filed with the Securities and Exchange Commission within 120 days after the end of the fiscal year covered by this Annual Report on Form 10 - K and portions of such are incorporated by reference into Part III of this Annual Report on Form 10 - K .