SC 13G

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

SCHEDULE 13G

Under the Securities Exchange Act of 1934

(Amendment No.     )*

Cabaletta Bio, Inc.

(Name of Issuer)

Class A Common Stock, par value $0.00001

(Title of Class of Securities)

12674W109

(CUSIP Number)

October 29, 2019

(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)

* The remainder of this cover page shall be filled out for a reporting person’s initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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CUSIP No. 12674W109

1	Name of reporting persons.    Boxer Capital, 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-
	6	Shared voting power.   1,566,361
	7	Sole dispositive power.    -0-
	8	Shared dispositive power.   1,566,361

9	Aggregate amount beneficially owned by each reporting person.    1,566,361
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).    9.1%*
12	Type of reporting person    OO

* Based on 17,143,335 shares of common stock outstanding after the Issuer’s initial public offering, as set forth in the Issuer’s final prospectus filed with the SEC on October 25, 2019 (the “Final Prospectus”), assuming no exercise of the underwriters’ option to purchase an additional 1,020,000 shares of common stock within 30 days of the date of the Final Prospectus.

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CUSIP No. 12674W109

1	Name of reporting persons.   Boxer Asset Management Inc.
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.   Bahamas

NUMBER OF SHARES   BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH:	5	Sole voting power.    -0-
	6	Shared voting power.   1,566,361
	7	Sole dispositive power.    -0-
	8	Shared dispositive power.   1,566,361

9	Aggregate amount beneficially owned by each reporting person.    1,566,361
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).    9.1%*
12	Type of reporting person    CO

* Based on 17,143,335 shares of common stock outstanding after the Issuer’s initial public offering, as set forth in the Final Prospectus, assuming no exercise of the underwriters’ option to purchase an additional 1,020,000 shares of common stock within 30 days of the date of the Final Prospectus.

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CUSIP No. 12674W109

1	Name of reporting persons.   Joe Lewis
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 Kingdom

NUMBER OF SHARES   BENEFICIALLY   OWNED BY EACH REPORTING PERSON WITH:	5	Sole voting power.    -0-
	6	Shared voting power.   1,566,361
	7	Sole dispositive power.    -0-
	8	Shared dispositive power.   1,566,361

9	Aggregate amount beneficially owned by each reporting person.    1,566,361
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).    9.1%*
12	Type of reporting person    IN

* Based on 17,143,335 shares of common stock outstanding after the Issuer’s initial public offering, as set forth in the Final Prospectus, assuming no exercise of the underwriters’ option to purchase an additional 1,020,000 shares of common stock within 30 days of the date of the Final Prospectus.

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CUSIP No. 12674W109

1	Name of reporting persons.   MVA Investors, 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.   89,414
	6	Shared voting power.   -0-
	7	Sole dispositive power.   89,414
	8	Shared dispositive power.   -0-

9	Aggregate amount beneficially owned by each reporting person.   89,414
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).    0.5%*
12	Type of reporting person    OO

* Based on 17,143,335 shares of common stock outstanding after the Issuer’s initial public offering, as set forth in the Final Prospectus, assuming no exercise of the underwriters’ option to purchase an additional 1,020,000 shares of common stock within 30 days of the date of the Final Prospectus.

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Item 1(a). Name of Issuer:

Cabaletta Bio, Inc. (the “Issuer”)

Item 1(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 jointly filed by Boxer Capital, LLC (“Boxer Capital”), Boxer Asset Management Inc. (“Boxer Management”), MVA Investors, LLC (“MVA Investors”) and Joe Lewis (together with Boxer Capital, Boxer Management and MVA Investors, the “Reporting Persons”). Boxer Management is the managing member and majority owner of Boxer Capital. Joe Lewis is the sole indirect beneficial owner of and controls Boxer Management. MVA Investors is the independent, personal investment vehicle of certain employees of Boxer Capital, and is controlled by employees of Boxer Capital that are members of MVA Investors. As such, MVA Investors is not controlled by any of the other Reporting Persons.

Item 2(b). Address of Principal Business Office, or, if none, Residence:

The principal business address of Boxer Capital and MVA Investors is: 11682 El Camino Real, Suite 320, San Diego, CA 92130. The principal business address of Boxer Management and Joe Lewis is: Cay House, EP Taylor Drive N7776, Lyford Cay, New Providence, Bahamas.

Item 2(c). Citizenship:

Boxer Capital is a limited liability company organized under the laws of Delaware. Boxer Management is a corporation organized under the laws of the Bahamas. MVA Investors is a limited liability company organized under the laws of Delaware. Joe Lewis is a citizen of the United Kingdom.

Item 2(d). Title of Class of Securities:

Common stock, par value $0.00001 (the “Common Stock”)

Item 2(e). CUSIP Number:

12674W109

Item 3. If this statement is filed pursuant to Rule 13d-1(b), or 13d-2(b) or (c), check whether the person filing is a:

Not applicable.

Item 4. Ownership.

(a) Amount beneficially owned:

Boxer Capital, Boxer Management and Joe Lewis beneficially own 1,566,361 shares of Common Stock. MVA Investors beneficially owns 89,414 shares of Common Stock. The Reporting Persons beneficially own, in the aggregate, 1,655,755 shares of Common Stock.

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(b) Percent of class:

The number of shares of Common Stock beneficially owned by Boxer Capital, Boxer Management and Joe Lewis represent 9.1% of the Issuer’s outstanding Common Stock. The number of shares of Common Stock beneficially owned by MVA Investors represent 0.5% of the Issuer’s outstanding Common Stock. The number of shares of Common Stock beneficially owned by the Reporting Persons represent, in the aggregate, 9.7% of the Issuer’s outstanding Common Stock. All percentages are based on 17,143,335 shares of common stock outstanding after the Issuer’s initial public offering, assuming no exercise of the underwriters’ option to purchase an additional 1,020,000 shares of common stock within 30 days of the date of the Final Prospectus.

(c) Number of shares as to which such person has:

(i) Sole power to vote or to direct the vote:

MVA Investors has the sole power to vote or to direct the vote of the 89,414 shares of Common Stock it beneficially owns.

(ii) Shared power to vote or to direct the vote:

Boxer Capital, Boxer Asset Management and Joe Lewis have shared power to vote or to direct the vote of the 1,566,361 shares of Common Stock they beneficially own.

(iii) Sole power to dispose or to direct the disposition of:

MVA Investors has the sole power to dispose or to direct the disposition of the 89,414 shares of Common Stock it beneficially owns.

(iv) Shared power to dispose or to direct the disposition of:

Boxer Capital, Boxer Asset Management and Joe Lewis have shared power to dispose or to direct the disposition of the 1,566,361 shares of Common Stock they beneficially own.

Item 5. Ownership of Five Percent or Less of a Class.

Not applicable.

Item 6. Ownership of More than Five Percent on Behalf of Another Person.

Only Boxer Capital has the right to receive dividends and the proceeds from the sale of the shares of Common Stock held by Boxer Capital. Only MVA Investors has the right to receive dividends and the proceeds from the sale of the shares of Common Stock held by MVA Investors.

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.

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Item 10. Certification.

(c) 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.

Exhibits

1 Joint Filing Agreement, dated November 7, 2019, among the Reporting Persons.

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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.

November 7, 2019

BOXER CAPITAL, LLC
By:	/s/ Aaron I. Davis
Name:	Aaron I. Davis
Title:	Chief Executive Officer
BOXER ASSET MANAGEMENT INC.
By:	/s/ Jason Callender
Name:	Jason Callender
Title:	Director
JOSEPH C. LEWIS
/s/ Joseph C. Lewis
Joseph C. Lewis, Individually
MVA INVESTORS, LLC
By:	/s/ Aaron I. Davis
Name:	Aaron I. Davis
Title:	Authorized Signatory

EX-99.1

JOINT FILING AGREEMENT

The undersigned hereby agree that the Statement on Schedule 13G with respect to the Class A common stock of Cabaletta Bio, Inc., par value $0.00001, is, and any amendments thereto executed by each of us shall be, filed on behalf of each of us pursuant to and in accordance with the provisions of Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended, and that this Agreement shall be included as an Exhibit to the Schedule 13G and each such amendment. Each of the undersigned agrees to be responsible for the timely filing of the Schedule 13G and any amendments thereto, and for the completeness and accuracy of the information concerning itself contained therein. This Agreement may be executed in any number of counterparts, all of which taken together shall constitute one and the same instrument.

IN WITNESS WHEREOF, the undersigned have executed this Agreement as of November 7, 2019.

BOXER CAPITAL, LLC
By:	/s/ Aaron I. Davis
Name:	Aaron I. Davis
Title:	Chief Executive Officer
BOXER ASSET MANAGEMENT INC.
By:	/s/ Jason Callender
Name:	Jason Callender
Title:	Director
JOSEPH C. LEWIS
/s/ Joseph C. Lewis
Joseph C. Lewis, Individually
MVA INVESTORS, LLC
By:	/s/ Aaron I. Davis
Name:	Aaron I. Davis
Title:	Authorized Signatory