[SCHEDULE 13G] Cabaletta Bio, Inc. SEC Filing

Rhea-AI Filing Summary

Cabaletta Bio (CABA) filed a Schedule 13G disclosing that Citadel Advisors LLC, Citadel Securities LLC, related entities and founder Kenneth Griffin collectively own 4,818,560 common shares, or approximately 5.2 % of the company’s 92.94 million shares outstanding.

Citadel Advisors–managed funds hold 4,687,280 shares, while market-making affiliate Citadel Securities holds 131,280 shares. All voting and dispositive power is reported as shared; none of the reporting persons has sole authority. The 5 % threshold was reached on 12 June 2025, triggering the filing. No additional financial results, risk factors, or strategic initiatives are included.

Positive

• Citadel affiliates and Kenneth Griffin disclosed a 5.2 % stake (4.82 million shares) in Cabaletta Bio, introducing a high-profile institutional holder.

Negative

• None.

Insights

Equity Ownership Analyst

TL;DR: Citadel now holds 5.2 % of CABA; passive stake, limited immediate impact.

Citadel Advisors and affiliated vehicles have crossed the 5 % beneficial-ownership line, requiring this Schedule 13G. The filing is made under Rule 13d-1(c), indicating a passive, non-activist position. All 4.82 million shares are subject to shared voting and dispositive power, suggesting coordinated but not unilateral decision-making. While the entrance of a sophisticated hedge fund may improve trading liquidity and spark investor attention, the document contains no plans for operational or governance changes. Consequently, the stake is noteworthy but does not materially alter Cabaletta Bio’s near-term fundamentals or control structure.

Corporate Governance Specialist

TL;DR: Governance framework unchanged; monitor for any shift to 13D.

Schedule 13G signifies a passive ownership intent. The group’s 5.2 % holding is below thresholds that typically secure board seats or trigger anti-takeover provisions. Zero sole voting power means no single entity can act independently, limiting influence on strategic decisions. Should the group increase its position or convert to a Schedule 13D, governance dynamics would warrant reassessment. For now, the filing is informational rather than transformational, keeping corporate control firmly with existing management and shareholders.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Cabaletta Bio, Inc.

(Name of Issuer)

Common stock, par value $0.00001 per share (the "Shares")

(Title of Class of Securities)

12674W109

(CUSIP Number)

06/12/2025

(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)

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel Advisors LLC

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 4,687,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 4,687,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

4,687,280.00

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)

5.0 %

12

Type of Reporting Person (See Instructions)

IA, HC, OO

Comment for Type of Reporting Person: The percentages reported in this Schedule 13G are based upon 92,943,101 Shares outstanding comprised of (i) 89,943,101 Shares outstanding as of immediately following the issuer's public offering (according to the issuer's Prospectus Supplement as filed with the Securities and Exchange Commission on June 12, 2025), and (ii) 3,000,000 Shares issuable upon conversion of certain warrants held by affiliates of the reporting persons. Except as described in the preceding sentence, all Shares for the holdings of the reporting persons reported in this Schedule 13G are as of the opening of the market on June 20, 2025.

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel Advisors Holdings 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 4,687,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 4,687,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

4,687,280.00

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)

5.0 %

12

Type of Reporting Person (See Instructions)

HC, PN

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel GP LLC

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 4,687,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 4,687,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

4,687,280.00

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)

5.0 %

12

Type of Reporting Person (See Instructions)

HC, OO

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel Securities LLC

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 131,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 131,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

131,280.00

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)

0.1 %

12

Type of Reporting Person (See Instructions)

BD, OO

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel Securities Group 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 131,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 131,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

131,280.00

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)

0.1 %

12

Type of Reporting Person (See Instructions)

HC, PN

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Citadel Securities GP LLC

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 131,280.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 131,280.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

131,280.00

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)

0.1 %

12

Type of Reporting Person (See Instructions)

HC, OO

SCHEDULE 13G

CUSIP No. 12674W109

1

Names of Reporting Persons

Kenneth Griffin

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

UNITED STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 0.00
6	Shared Voting Power 4,818,560.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 4,818,560.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

4,818,560.00

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)

5.2 %

12

Type of Reporting Person (See Instructions)

HC, IN

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, PA, 19104
Item 2.
(a)	Name of person filing: This Schedule 13G is being jointly filed by Citadel Advisors LLC ("Citadel Advisors"), Citadel Advisors Holdings LP ("CAH"), Citadel GP LLC ("CGP"), Citadel Securities LLC ("Citadel Securities"), Citadel Securities Group LP ("CALC4"), Citadel Securities GP LLC ("CSGP") and Mr. Kenneth Griffin (collectively with Citadel Advisors, CAH, CGP, Citadel Securities, CALC4 and CSGP, the "Reporting Persons") with respect to the Shares of the above-named issuer owned by Citadel Multi-Strategy Equities Master Fund Ltd., a Cayman Islands company ("CM"), and Citadel Securities. Such owned Shares may include other instruments exercisable for or convertible into Shares. Citadel Advisors is the portfolio manager for CM. CAH is the sole member of Citadel Advisors. CGP is the general partner of CAH. CALC4 is the non-member manager of Citadel Securities. CSGP is the general partner of CALC4. Mr. Griffin is the President and Chief Executive Officer of CGP, and owns a controlling interest in CGP and CSGP. The filing of this statement shall not be construed as an admission that any of the Reporting Persons is the beneficial owner of any securities covered by the statement other than the securities actually owned by such person (if any).
(b)	Address or principal business office or, if none, residence: The address of each of CSGP, Citadel Securities and CALC4 is 830 Brickell Plaza, Miami, Florida 33131. The address of the other Reporting Persons is Southeast Financial Center, 200 S. Biscayne Blvd., Suite 3300, Miami, Florida 33131.
(c)	Citizenship: Each of Citadel Advisors, CGP, Citadel Securities and CSGP is organized as a limited liability company under the laws of the State of Delaware. Each of CALC4 and CAH is organized as a limited partnership under the laws of the State of Delaware. Mr. Griffin is a U.S. citizen.
(d)	Title of class of securities: Common stock, par value $0.00001 per share (the "Shares")
(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: 1. Each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC may be deemed to beneficially own 4,687,280 Shares. 2. Citadel Securities LLC may be deemed to beneficially own 131,280 Shares. 3. Each of Citadel Securities Group LP and Citadel Securities GP LLC may be deemed to beneficially own 131,280 Shares. 4. Mr. Griffin may be deemed to beneficially own 4,818,560 Shares.
(b)	Percent of class: 1. The number of Shares that each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC may be deemed to beneficially own constitutes 5.0% of the Shares outstanding. 2. The number of Shares that Citadel Securities LLC may be deemed to beneficially own constitutes 0.1% of the Shares outstanding. 3. The number of Shares that each of Citadel Securities Group LP and Citadel Securities GP LLC may be deemed to beneficially own constitutes 0.1% of the Shares outstanding. 4. The number of Shares that Mr. Griffin may be deemed to beneficially own constitutes 5.2% of the Shares outstanding.
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 1. Each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC: 0 2. Citadel Securities LLC: 0 3. Each of Citadel Securities Group LP and Citadel Securities GP LLC: 0 4. Mr. Griffin: 0
	(ii) Shared power to vote or to direct the vote: 1. Each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC: 4,687,280 2. Citadel Securities LLC: 131,280 3. Each of Citadel Securities Group LP and Citadel Securities GP LLC: 131,280 4. Mr. Griffin: 4,818,560
	(iii) Sole power to dispose or to direct the disposition of: 1. Each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC: 0 2. Citadel Securities LLC: 0 3. Each of Citadel Securities Group LP and Citadel Securities GP LLC: 0 4. Mr. Griffin: 0
	(iv) Shared power to dispose or to direct the disposition of: 1. Each of Citadel Advisors LLC, Citadel Advisors Holdings LP and Citadel GP LLC: 4,687,280 2. Citadel Securities LLC: 131,280 3. Each of Citadel Securities Group LP and Citadel Securities GP LLC: 131,280 4. Mr. Griffin: 4,818,560
Item 5.	Ownership of 5 Percent or Less of a Class.
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.

Citadel Advisors LLC

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Citadel Advisors Holdings LP

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Citadel GP LLC

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Citadel Securities LLC

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Citadel Securities Group LP

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Citadel Securities GP LLC

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, Authorized Signatory
Date:	06/20/2025

Kenneth Griffin

Signature:	/s/ Seth Levy
Name/Title:	Seth Levy, attorney-in-fact*
Date:	06/20/2025

Comments accompanying signature: * Seth Levy is signing on behalf of Kenneth Griffin as attorney-in-fact pursuant to a power of attorney previously filed with the Securities and Exchange Commission, and hereby incorporated by reference herein. The power of attorney was filed as an attachment to a filing by Citadel Advisors LLC on Schedule 13G for Allakos Inc. on October 13, 2023.

Exhibit Information

Exhibit 99.1 - Joint Filing Agreement

FAQ

How many Cabaletta Bio (CABA) shares did Citadel entities report owning?

The Schedule 13G states Citadel-managed funds hold 4,687,280 shares, with shared voting and dispositive power.

What percentage of Cabaletta Bio’s outstanding stock does Citadel’s position represent?

Citadel affiliates reported ownership of approximately 5.0 %; including Kenneth Griffin’s aggregated amount, the stake is 5.2 %.

Who is identified as the ultimate controlling person in the 13G filing?

The filing names Kenneth Griffin as the control person with a beneficial interest in 4,818,560 shares.

Did any Citadel entity report sole voting power over CABA shares?

No. Each reporting person disclosed 0 shares of sole voting power; all authority is shared.

What date triggered the Schedule 13G requirement for Cabaletta Bio?

The event date is June 12, 2025, when the ownership threshold exceeded 5 %.