SCHEDULE 13G/A 0001487989 XXXXXXXX LIVE 3 Common Stock, par value $0.00001 per share 06/30/2025 0001759138 Cabaletta Bio, Inc. 12674W109 2929 ARCH STREET SUITE 600 PHILADELPHIA PA 19104 Rule 13d-1(d) Nichtberger Steven X1 3034499.00 0.00 3034499.00 0.00 3034499.00 3.3 IN Cabaletta Bio, Inc. 2929 ARCH STREET, SUITE 600, PHILADELPHIA, PENNSYLVANIA, 19104. This Schedule 13G is being filed on behalf of Steven Nichtberger (the "Reporting Person"). c/o Cabaletta Bio, Inc. 2929 Arch Street, Suite 600 Philadelphia, PA 19104 United States of America Y As of June 30, 2025, the Reporting Person beneficially owned an aggregate of 3,034,499 shares of the Common Stock, par value $0.00001 per share ("Common Stock") of Cabaletta Bio, Inc. (the "Issuer"), representing approximately 3.3% of the outstanding Common Stock, consisting of (a) 986,483 shares of Common Stock held directly by Dr. Nichtberger, representing approximately 1.1% of the outstanding Common Stock, (b) 363,000 shares of Common Stock held directly by the 2017 Nichtberger Family Trust, representing approximately 0.4% of the outstanding Common Stock, and (c) 1,685,016 shares of Common Stock underlying options held directly by Dr. Nichtberger that are exercisable within 60 days of June 30, 2025, representing approximately 1.8% of the outstanding Common Stock. Dr. Nichtberger is a trustee of the 2017 Nichtberger Family Trust. The foregoing percentages were calculated based on (a) 91,451,899 shares of the Issuer's Common Stock outstanding as of June 30, 2025 plus (b) 1,685,016 shares of Common Stock underlying stock options held by Dr. Nichtberger that are exercisable within 60 days of June 30, 2025, which are deemed outstanding pursuant to SEC Rule 13-3(d)(1)(i). 3,034,499 0 3,034,499 0 Y Y Y Y Y Y Nichtberger Steven /s/ Steven Nichtberger 08/11/2025