Caballeta Bio (CABA) stake: Bain Capital Life Sciences reports 9.99% position

Rhea-AI Filing Summary

Bain Capital Life Sciences Opportunities III, L.P. reports beneficial ownership of common stock of Caballeta Bio, Inc.. As of June 30, 2026, it held 7,709,188 shares of common stock, a warrant to purchase up to 7,500,000 shares, and a pre-funded warrant to purchase up to 6,000,000 shares.

Because of a 9.99% Beneficial Ownership Blocker, Bain Capital Life Sciences may be deemed to beneficially own 17,241,445 shares, or 9.99% of Caballeta Bio’s common stock, based on 163,054,796 shares outstanding plus 9,532,257 shares issuable under the warrants within the blocker. Voting and dispositive power over these securities is shared among affiliated Bain Capital Life Sciences entities.

Key Figures

Beneficial ownership: 17,241,445 shares Ownership percentage: 9.99% Common shares held: 7,709,188 shares +4 more

7 metrics

Beneficial ownership 17,241,445 shares Shares of Caballeta Bio common stock deemed beneficially owned by Bain Capital Life Sciences

Ownership percentage 9.99% Percentage of Caballeta Bio common stock beneficially owned, limited by Beneficial Ownership Blocker

Common shares held 7,709,188 shares Caballeta Bio common stock directly held as of June 30, 2026

Warrant shares 7,500,000 shares Shares of common stock issuable upon exercise of a warrant held by Bain Capital Life Sciences

Pre-funded warrant shares 6,000,000 shares Shares of common stock issuable upon exercise of a pre-funded warrant

Shares outstanding 163,054,796 shares Caballeta Bio common stock outstanding as of May 7, 2026, used for ownership calculation

Shares issuable within blocker 9,532,257 shares Shares issuable upon partial exercise of the warrant and pre-funded warrant under the 9.99% blocker

Key Terms

Beneficial Ownership Blocker, pre-funded warrant, beneficially own, dispositive power, +1 more

5 terms

Beneficial Ownership Blocker regulatory

"The Reporting Person is prohibited from exercising the warrant... (the "Beneficial Ownership Blocker")."

A beneficial ownership blocker is a legal or structural device that prevents a shareholder from being treated as the ultimate owner of enough shares to trigger control, reporting, or voting thresholds. Think of it like a speed bump that stops an investor from reaching a stake size that would force corporate disclosure or change control rights. Investors care because it affects who controls the company, how shares vote, regulatory filings, takeover risk and therefore potential value or liquidity of their holdings.

pre-funded warrant financial

"a pre-funded warrant to purchase up to 6,000,000 shares of Common Stock."

A pre-funded warrant is a financial instrument that gives the holder the right to buy shares of a company's stock at a set price, with most of the purchase cost already paid upfront. It functions like a nearly fully paid option, allowing investors to secure shares quickly while minimizing the amount of additional money they need to invest later. This helps investors gain ownership rights efficiently, often used to avoid certain regulatory restrictions or to prepare for future stock purchases.

beneficially own regulatory

"the Reporting Person would beneficially own more than 9.99% of the total number of shares."

Beneficially own means having the economic rights and risks of a security—such as the right to receive dividends, sell the shares, or profit from price changes—whether or not your name appears on the official share register. Think of it like renting a car: you use it and reap the benefits even if the title lists someone else. Investors care because beneficial ownership determines who truly controls value, must be disclosed under securities rules, and can signal potential influence or trading activity that affects a stock’s price.

dispositive power financial

"Shared Dispositive Power 17,241,445.00"

Dispositive power is the authority to decide the final outcome of an asset, legal claim, contract, or corporate action — in effect the power to dispose of or resolve something. For investors it matters because whoever holds that authority can determine who gets paid, who controls an asset or vote, and how risks and returns are allocated; think of it like holding the key that lets you lock in the winner or loser in a deal.

Schedule 13d-3 regulatory

"Accordingly, pursuant to Rule 13d-3 of the Act, the Reporting Person may be deemed..."

FAQ

What percentage of CABA common stock does Bain Capital Life Sciences report owning?

Bain Capital Life Sciences reports beneficial ownership of 9.99% of Caballeta Bio’s common stock, representing 17,241,445 shares, including shares issuable upon partial exercise of its warrants, subject to a 9.99% Beneficial Ownership Blocker.

How many CABA shares and warrants does Bain Capital Life Sciences hold?

As of June 30, 2026, Bain Capital Life Sciences held 7,709,188 common shares, a warrant for up to 7,500,000 shares, and a pre-funded warrant for up to 6,000,000 shares, giving it substantial potential additional equity exposure.

What is the Beneficial Ownership Blocker in Bain Capital’s CABA position?

The Beneficial Ownership Blocker prevents Bain Capital Life Sciences from exercising its warrants if doing so would raise its beneficial ownership above 9.99% of Caballeta Bio’s outstanding common stock immediately after exercise.

On what share count is Bain Capital’s 9.99% CABA ownership based?

The reported 9.99% ownership is based on 163,054,796 shares of Caballeta Bio common stock outstanding as of May 7, 2026, plus 9,532,257 shares issuable upon partial warrant and pre-funded warrant exercise within the Beneficial Ownership Blocker.

Which Bain Capital entities share control over the CABA securities?

Voting and dispositive power over the Caballeta Bio securities is shared among Bain Capital Life Sciences Opportunities III, L.P. and related Bain Capital Life Sciences entities, including their general partners and manager, which may be deemed to share such powers.

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Available on EDGAR 08/14/2026 - 07:00 AM Accepted by SEC EDGAR 08/14/2026 - 07:00 AM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 2)

CABALETTA BIO, INC.

(Name of Issuer)

Common Stock, $0.00001 par value per share

(Title of Class of Securities)

12674W109

(CUSIP Number)

06/30/2026

(Date of Event Which Requires Filing of this Statement)

Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

Rule 13d-1(b)

Rule 13d-1(c)

Rule 13d-1(d)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 12674W109

1

Names of Reporting Persons

Bain Capital Life Sciences Opportunities III, LP

2

Check the appropriate box if a member of a Group (see instructions)

(a)
(b)

3

Sec Use Only

4

Citizenship or Place of Organization

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 17,241,445.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 17,241,445.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

17,241,445.00

10

Check box if the aggregate amount in row (9) excludes certain shares (See Instructions)

11

Percent of class represented by amount in row (9)

9.99 %

12

Type of Reporting Person (See Instructions)

PN

SCHEDULE 13G

Item 1.
(a)	Name of issuer: CABALETTA BIO, INC.
(b)	Address of issuer's principal executive offices: 2929 Arch Street, Suite 600, Philadelphia, Pennsylvania 19104
Item 2.
(a)	Name of person filing: 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.
(b)	Address or principal business office or, if none, residence: The principal business address for each of the Bain Capital Life Sciences Entities is 200 Clarendon Street, Boston, Massachusetts 02116.
(c)	Citizenship: Each of the Bain Capital Life Sciences Entities is organized under the laws of the State of Delaware.
(d)	Title of class of securities: Common Stock, $0.00001 par value per share
(e)	CUSIP No.: 12674W109
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: As of the close of business on June 30, 2026, the Reporting Person held 7,709,188 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 17,241,445 shares of Common Stock. The percentage of the outstanding shares of Common Stock held by the Reporting Person is based on (i) 163,054,796 shares of Common Stock outstanding as of May 7, 2026, as reported by the Issuer in its Quarterly Report on Form 10-Q for the quarter ended March 31, 2026, filed with the Securities and Exchange Commission on May 14, 2026 and (ii) 9,532,257 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.
(b)	Percent of class: See Item 4(a) hereof.
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 0
	(ii) Shared power to vote or to direct the vote: See Item 4(a) hereof.
	(iii) Sole power to dispose or to direct the disposition of: 0
	(iv) Shared power to dispose or to direct the disposition of: See Item 4(a) hereof.
Item 5.	Ownership of 5 Percent or Less of a Class.
	Not Applicable
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	Not Applicable
Item 8.	Identification and Classification of Members of the Group.
	Not Applicable
Item 9.	Notice of Dissolution of Group.
	Not Applicable

Item 10.

Certifications:

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.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Bain Capital Life Sciences Opportunities III, LP

Signature:	/s/ Andrew Hack
Name/Title:	Andrew Hack, Partner of Bain Capital Life Sciences Investors, LLC
Date:	08/14/2026