UNITED STATES SECURITIES AND EXCHANGE COMMISSION WASHINGTON, DC 20549 Securities registered pursuant to Section 12(b) of the Act: 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, 2021 OR ☐ 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: 001-40631 Caribou Biosciences, Inc. (Exact Name of Registrant as Specified in ...