UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 FORM 10-Q QUARTERLY REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 ☒ For the quarterly period ended September 30, 2023 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 001-40791 2seventy bio, Inc. (Exact name of registrant as specified in its charter) Delaware 86-3658454 (State or other ...