Ownership Submission

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549
INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934 or Section 30(h) of the Investment Company Act of 1940

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3235-0104

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(Print or Type Responses)

1. Name and Address of Reporting Person ^*

5AM Ventures V, L.P.

2. Date of Event Requiring Statement (Month/Day/Year)
10/24/2019

3. Issuer Name and Ticker or Trading Symbol
Cabaletta Bio, Inc. [CABA]

(Last)

(First)

(Middle)

501 2ND STREET, SUITE 350

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

_____ Director	__X__ 10% Owner
_____ Officer (give title below)	_____ Other (specify below)

5. If Amendment, Date Original Filed(Month/Day/Year)

(Street)

SAN FRANCISCO, CA 94107

6. Individual or Joint/Group Filing(Check Applicable Line)
___ Form filed by One Reporting Person
_X_ Form filed by More than One Reporting Person

(City)

(State)

(Zip)

Table I - Non-Derivative Securities Beneficially Owned

1.Title of Security
(Instr. 4)

2. Amount of Securities Beneficially Owned
(Instr. 4)

3. Ownership Form: Direct (D) or Indirect (I)
(Instr. 5)

4. Nature of Indirect Beneficial Ownership
(Instr. 5)

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.			SEC 1473 (7-02)
	Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB control number.

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)

1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivative Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security		6. Nature of Indirect Beneficial Ownership (Instr. 5)
Series A Preferred Stock	⁽¹⁾	⁽¹⁾	Common Stock	3,146,551	$ ⁽¹⁾	I ⁽²⁾ ⁽³⁾	See footnotes ⁽²⁾ ⁽³⁾
Series B Preferred Stock	⁽¹⁾	⁽¹⁾	Common Stock	1,675,532	$ ⁽¹⁾	I ⁽²⁾ ⁽³⁾	See footnotes ⁽²⁾ ⁽³⁾

Reporting Owners

Reporting Owner Name / Address	Relationships
Reporting Owner Name / Address	Director	10% Owner	Officer	Other
5AM Ventures V, L.P. 501 2ND STREET, SUITE 350 SAN FRANCISCO, CA 94107		X
5AM Partners V, LLC 501 2ND STREET, SUITE 350 SAN FRANCISCO, CA 94107		X
PARMAR KUSH 501 2ND STREET, SUITE 350 SAN FRANCISCO, CA 94107		X
ROCKLAGE SCOTT M 501 2ND STREET, SUITE 350 SAN FRANCISCO, CA 94107		X
Schwab Andrew J. 501 2ND STREET, SUITE 350 SAN FRANCISCO, CA 94107		X

Signatures

5AM VENTURES V, L.P. By: 5AM Partners V, LLC, its General Partner By: /s/ Scott M. Rocklage Managing Member		10/24/2019
^**Signature of Reporting Person		Date
5AM PARTNERS V, LLC By: /s/ Scott M. Rocklage Managing Member		10/24/2019
^**Signature of Reporting Person		Date
/s/ Kush Parmar		10/24/2019
^**Signature of Reporting Person		Date
/s/ Scott Rocklage		10/24/2019
^**Signature of Reporting Person		Date
/s/ Andrew J. Schwab		10/24/2019
^**Signature of Reporting Person		Date

Explanation of Responses:

*	If the form is filed by more than one reporting person, see Instruction 5(b)(v).
**	Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
(1)	These shares of Series A Preferred Stock and Series B Preferred Stock (collectively, "Preferred Stock") will automatically be converted into shares of the Issuer's Common Stock ("Common Stock"), on a 1-for-1.5 basis, immediately prior to the completion of the Issuer's initial public offering. The Preferred Stock has no expiration date.
(2)	Shares are held directly by 5AM Ventures V, L.P. ("5AM V").
(3)	5AM Partners V, LLC ("5AM Partners") is the sole general partner of 5AM V. Kush Parmar, Andrew J. Schwab and Dr. Scott M. Rocklage are managing members of 5AM Partners and may be deemed to have shared voting and investment power over the shares beneficially owned by 5AM V. Each of 5AM Partners, Mr. Parmar, Mr. Schwab and Dr. Rocklage disclaims beneficial ownership of the shares of Preferred Stock held directly by 5AM V, except to the extent of its or his pecuniary interest therein.

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