SCHEDULE 13G/A 0001950105 XXXXXXXX LIVE 1 Common Stock, $0.00001 par value per share 06/30/2025 0001759138 CABALETTA BIO, INC. 12674W109 2929 Arch Street Suite 600 Philadelphia PA 19104 Rule 13d-1(c) Bain Capital Life Sciences Opportunities III, LP DE 0 9677125 0 9677125 9677125 N 9.99 PN CABALETTA BIO, INC. 2929 Arch Street, Suite 600, Philadelphia, Pennsylvania 19104 This Schedule 13G is being filed by Bain Capital Life Sciences Opportunities III, L.P., a Delaware limited partnership (the "Reporting Person"). Bain Capital Life Sciences Investors, LLC, a Delaware limited liability company ("BCLSI"), is the manager of Bain Capital Life Sciences III General Partner, LLC, a Delaware limited liability company ("BCLS Fund III GP"), which is the general partner of Bain Capital Life Sciences Fund III, L.P., a Delaware limited partnership ("BCLS Fund III"), which is the sole member of Bain Capital Life Sciences Opportunities III GP, LLC, a Delaware limited liability company (together with the Reporting Person, BCLSI, BCLS Fund III GP and BCLS Fund III, the "Bain Capital Life Sciences Entities"), which is the general partner of the Reporting Person. As a result, BCLSI may be deemed to share voting and dispositive power with respect to the securities held by the Reporting Person. The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116. Each of the Bain Capital Life Sciences Entities is organized under the laws of the State of Delaware. Y As of the close of business on June 30, 2025, the Reporting Person held 4,260,888 shares of Common Stock, a warrant to purchase up to 7,500,000 shares of Common Stock and a pre-funded warrant to purchase up to 6,000,000 shares of Common Stock. The Reporting Person is prohibited from exercising the warrant to purchase Common Stock or pre-funded warrant to purchase Common Stock if, as a result of such exercise, the Reporting Person would beneficially own more than 9.99% of the total number of shares of Common Stock then issued and outstanding immediately after giving effect to the exercise (the "Beneficial Ownership Blocker"). Accordingly, pursuant to Rule 13d-3 of the Act, the Reporting Person may be deemed to beneficially own 9.99% of the outstanding shares of Common Stock, representing 9,677,125 shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Person is based on (i) 91,451,899 shares of Common Stock outstanding as of June 30, 2025, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended June 30, 2025, filed with the Securities and Exchange Commission on August 7, 2025 and (ii) 5,416,237 shares of Common Stock issuable upon the partial exercise of the warrant and/or pre-funded warrant held by the Reporting Person, which reflects the Beneficial Ownership Blocker. See Item 4(a) hereof. 0 See Item 4(a) hereof. 0 See Item 4(a) hereof. Y Y Y Y Y N By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer of the securities and were not acquired and are not held in connection with or as a participant in any transaction having that purpose or effect, other than activities solely in connection with a nomination under ?? 240.14a-11. Bain Capital Life Sciences Opportunities III, LP /s/ Andrew Hack 08/14/2025