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( Mark One ) 区 ☐ 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 Delaware ΤΟ Commission File Number 001-39103 CABALETTA BIO , INC . ( State or other jurisdiction of incorporation or organization ) 2929 Arch Street , Suite 600 Philadelphia , PA ( Exact name of Registrant as specified in its Charter ) ( Address of principal executive offices ) Registrant's telephone number , including area code : ( 267 ) 759-3100 82-1685768 ( I.R.S. Employer Identification No. ) 19104 ( Zip Code ) Securities registered pursuant to Section 12 ( b ) of the Act : Title of each class Common Stock , par value $ 0.00001 per share Securities registered pursuant to Section 12 ( g ) of the Act : None Trading Symbol ( s ) CABA 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 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 Section 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 ㅁ ☐ 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 Act ) . Yes □ No ☑ As of June 30 , 2020 ( the last business day of the registrant's most recently completed second fiscal quarter ) , the aggregate market value of the registrant's common stock held by non - affiliates was approximately $ 214 million based on the last reported sale price of the registrant's common stock on the Nasdaq Global Select Market on June 30 , 2020 . The number of shares of registrant's Common Stock outstanding as of March 10 , 2021 was 24,062,775 . DOCUMENTS INCORPORATED BY REFERENCE Part III of this Annual Report on Form 10 - K incorporates by reference certain information from the registrant's definitive Proxy Statement for its 2021 annual meeting of shareholders , which the registrant intends to file pursuant to Regulation 14A with the Securities and Exchange Commission not later than 120 days after the registrant's fiscal year end of December 31 , 2020. Except with respect to information specifically incorporated by reference in this Form 10 - K , the Proxy Statement is not deemed to be filed as part of this Form 10 - K .