Sec Form 4 Filing - Singer Michael @ Cartesian Therapeutics, Inc. - 2023-12-22

FORM 4 Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934, Section 17(a) of the Public Utility Holding Company Act of 1935 or Section 30(h) of the Investment Company Act of 1940

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1. Name and Address of Reporting Person Singer Michael 2. Issuer Name and Ticker or Trading Symbol Cartesian Therapeutics, Inc. [ RNAC] 5. Relationship of Reporting Person(s) to Issuer (Check all applicable) X __ Director X __ 10% Owner _____ Officer (give title below) _____ Other (specify below) (Last) (First) (Middle) C/O CARTESIAN THERAPEUTICS, INC., 704 QUINCE ORCHARD ROAD 3. Date of Earliest Transaction (MM/DD/YY) 12/22/2023 (Street) GAITHERSBURG, MD20878 4. If Amendment, Date Original Filed (MM/DD/YY) 6. Individual or Joint/Group Filing (Check Applicable Line) __ X __ Form filed by One Reporting Person _____ Form filed by More than One Reporting Person (City) (State) (Zip)

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned 1.Title of Security (Instr. 3) 2. Transaction Date (MM/DD/YY) 2A. Deemed Execution Date, if any (MM/DD/YY) 3. Transaction Code (Instr. 8) 4. Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5) 5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4) 6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4) 7. Nature of Indirect Beneficial Ownership (Instr. 4) Code V Amount (A) or (D) Price Common Stock 01/02/2024 A( 1 ) 178,000 A $ 0 288,123 D Common Stock 197,663 I by trust ( 2 ) Common Stock 49,555 I as custodian ( 3 ) Common Stock 49,555 I as custodian ( 4 ) Common Stock 3,670 I by trust ( 5 ) Common Stock 29,212 I by trust ( 6 ) Common Stock 7,616 I by spouse ( 7 )

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned ( e.g. , puts, calls, warrants, options, convertible securities) 1. Title of Derivative Security (Instr. 3) 2. Conversion or Exercise Price of Derivative Security 3. Transaction Date (MM/DD/YY) 3A. Deemed Execution Date, if any (MM/DD/YY) 4. Transaction Code (Instr. 8) 5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4, and 5) 6. Date Exercisable and Expiration Date (MM/DD/YY) 7. Title and Amount of Underlying Securities (Instr. 3 and 4) 8. Price of Derivative Security (Instr. 5) 9. Number of Derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4) 10. Ownership Form of Derivative Security: Direct (D) or Indirect (I) (Instr. 4) 11. Nature of Indirect Beneficial Ownership (Instr. 4) Code V (A) (D) Date Exercisable Expiration Date Title Amount or Number of Shares Stock Option (Right to Buy) $ 0.6552 01/02/2024 A 228,000 ( 8 ) 01/01/2034 Common Stock 228,000 $ 0 228,000 D Series A Convertible Preferred Stock ( 9 ) 12/22/2023 G( 10 ) 3,000 ( 9 ) ( 9 ) Common Stock 3,000,000 $ 0 3,304.559 D Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 6,304,559 6,304.559 I by trust ( 2 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 11,316,263 11,316.263 I by trust ( 6 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 1,672,389 1,672.389 I by trust ( 6 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 210,152 210.152 I by trust ( 5 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 2,837,052 2,837.052 I as custodian ( 3 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 2,837,052 2,837.052 I as custodian ( 4 ) Series A Convertible Preferred Stock ( 9 ) ( 9 ) ( 9 ) Common Stock 436,066 436.066 I by spouse ( 7 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Singer Michael C/O CARTESIAN THERAPEUTICS, INC. 704 QUINCE ORCHARD ROAD GAITHERSBURG, MD20878 X X Signatures /s/ Matthew Bartholomae, Attorney-in-Fact for Michael Singer 01/04/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Each Restricted Stock Unit represents a contingent right to receive one share of common stock, which will vest in three equal annual installments so that the underlying shares will be fully vested on January 2, 2027. ( 2 )Shares are held in a trust for the benefit of the reporting person's spouse and children. The reporting person's spouse is a trustee of the trust. ( 3 )The shares are held for LS, the child of the reporting person, through a custodial account established pursuant to the Uniform Transfer to Minors Act ("UTMA") for which the reporting person serves as custodian. ( 4 )The shares are held for SS, the child of the reporting person, through a custodial account established pursuant to the UTMA for which the reporting person serves as custodian. ( 5 )Shares are held in a trust for which the reporting person is a beneficiary. The reporting person's spouse is a trustee of the trust. ( 6 )Shares are held in a trust for which the reporting person is a beneficiary. The reporting person is a trustee of the trust. ( 7 )Shares are held by the reporting person's spouse. ( 8 )This award will vest in 36 equal monthly installments, so that it shall be fully vested on the third anniversary of the date of grant. ( 9 )Following the approval by the Stockholders of the conversion of the Series A Non-Voting Convertible Preferred Stock of the Company ("Series A Preferred Stock") into shares of common stock of the Company ("Common Stock"), each share of Series A Preferred Stock will be convertible into 1,000 shares of Common Stock, subject to certain limitations, including that a holder of Series A Preferred Stock is prohibited from converting shares of Series A Preferred Stock into shares of Common Stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than 19.9% of the total number of shares of Common Stock issued and outstanding immediately after giving effect to such conversion. ( 10 )Bona fide gift to Yale University.

Remarks: Exhibit 24 - Power of Attorney

Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.

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