Cabaletta Bio Inc

Rule 10b5-1 Trading Plans

During the fiscal quarter ended on December 31, 2023, none of our directors and officers (as defined in Rule 16a-1(f) under the Exchange Act)adopted, modifiedor terminated a “Rule 10b5-1 trading arrangement” or “non-Rule 10b5-1 trading arrangement,” as those terms are defined in Item 408(a) of Regulation S-K, except as described in the table below:

• Denotes whether the trading plan is intended, when adopted, to satisfy the affirmative defense of Rule 10b5-1(c).

(1) Except as indicated by footnote, each trading arrangement permitted or permits transactions through and including the earlier to occur of (a) the completion of all purchases or sales or (b) the date listed in the table.

(2) Represents the modification, as described in Rule 10b5-1(c)(1)(iv) under the Exchange Act, of a written trading arrangement adopted on August 30, 2023 that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act.

(3) Represents the modification, as described in Rule 10b5-1(c)(1)(iv) under the Exchange Act, of a written trading arrangement adopted on June 8, 2023 that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act.

(4) Represents the modification, as described in Rule 10b5-1(c)(1)(iv) under the Exchange Act, of a written trading arrangement adopted on September 5, 2023 that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act.

(5) Represents the modification, as described in Rule 10b5-1(c)(1)(iv) under the Exchange Act, of a written trading arrangement adopted on August 24, 2023 that was intended to satisfy the affirmative defense conditions of Rule 10b5-1(c) under the Exchange Act.

Termination of 2023 At-The-Market Offering

On March 16, 2023, we entered into the 2023 Sales Agreement with the Sales Agent, pursuant to which we could, from time to time, sell shares of our common stock having an aggregate offering price of up to $100.0 million through the Sales Agent, as our agent, or the 2023 ATM Offering. In connection with the 2023 ATM Offering, we filed a prospectus, dated March 16, 2023, with the 2023 Registration Statement, or the 2023 ATM Prospectus. Between March 16, 2023 and March 21, 2024, we sold an aggregate of 5,018,969 shares of common stock pursuant to the 2023 Sales Agreement for aggregate gross proceeds of approximately $99.9 million, thereby selling the total amount available under the 2023 Sales Agreement. Pursuant to the terms of the 2023 Sales Agreement, effective March 21, 2024, or the Termination Date, we terminated the 2023 Sales Agreement, and we have discontinued all offers and sales of shares of our common stock under the 2023 Sales Agreement and the 2023 ATM Prospectus. All of the continuing obligations under the 2023 Sales Agreement will be terminated as of the Termination Date, other than those provisions which expressly survive termination as provided in the 2023 Sales Agreement. We are not subject to any termination penalties related to the termination of the 2023 Sales Agreement. A copy of the 2023 Sales Agreement was filed as Exhibit 1.2 to our Registration Statement on Form S-3 (File No. 333-270599) filed with the SEC on March 16, 2023.