Sec Form 4 Filing - Singer Michael @ Cartesian Therapeutics, Inc. - 2024-04-08

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 _____ 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) 04/08/2024 (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 04/08/2024 M 110,151 A 119,754 ( 2 ) D Common Stock 04/08/2024 M 377,208 A 383,796 ( 2 ) I by trust ( 3 ) Common Stock 04/08/2024 M 55,746 A 56,719 ( 2 ) I by trust ( 4 ) Common Stock 04/08/2024 M 7,005 A 7,127 ( 2 ) I by trust ( 5 ) Common Stock 04/08/2024 M 14,535 A 14,788 ( 2 ) I by spouse ( 6 ) Common Stock 04/08/2024 M 94,568 A 96,219 ( 2 ) I as custodian ( 7 ) Common Stock 04/08/2024 M 94,568 A 96,219 ( 2 ) I as custodian ( 8 )

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 Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 3,304.559 ( 1 ) ( 1 ) Common Stock 110,151 ( 2 ) ( 1 ) 0 D Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 11,316.263 ( 1 ) ( 1 ) Common Stock 377,208 ( 2 ) ( 1 ) 0 I by trust ( 3 ) Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 1,672.389 ( 1 ) ( 1 ) Common Stock 55,746 ( 2 ) ( 1 ) 0 I by trust ( 4 ) Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 210.152 ( 1 ) ( 1 ) Common Stock 7,005 ( 2 ) ( 1 ) 0 I by trust ( 5 ) Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 436.066 ( 1 ) ( 1 ) Common Stock 14,535 ( 2 ) ( 1 ) 0 I by spouse ( 6 ) Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 2,837.052 ( 1 ) ( 1 ) Common Stock 94,568 ( 2 ) ( 1 ) 0 I as custodian ( 7 ) Series A Non-Voting Convertible Preferred Stock ( 1 ) 04/08/2024 M 2,837.052 ( 1 ) ( 1 ) Common Stock 94,568 ( 2 ) ( 1 ) 0 I as custodian ( 8 ) Stock Option (Right to Buy) $ 19.656 ( 9 ) 01/01/2034 Common Stock 7,600 ( 2 ) 7,600 ( 2 ) D

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 Signatures /s/ Matthew Bartholomae, Attorney-in-Fact for Michael Singer 04/10/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )On March 27, 2024, the issuer held a special meeting of stockholders (the "Special Meeting"). At the Special Meeting, the stockholders of the issuer approved a 1-for-30 reverse stock split (the "Reverse Stock Split") of the issuer's common stock ("Common Stock") and the conversion of the issuer's Series A Non-Voting Convertible Preferred Stock ("Series A Preferred Stock") into shares of Common Stock. On April 8, 2024, the conversion of the majority of the shares of Series A Preferred Stock occurred automatically (the "Automatic Conversion"). The remaining shares of Series A Preferred Stock remain subject to certain beneficial ownership limitations described in the issuer's filings with the Securities and Exchange Commission. At the time of the Automatic Conversion, all of the reporting person's shares of Series A Preferred Stock converted into shares of Common Stock. ( 2 )On April 4, 2024, the issuer effected the Reverse Stock Split. The amount of securities reported on this Form 4 has been adjusted to reflect the Reverse Stock Split. ( 3 )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 this trust. ( 4 )Shares are held in a trust for which the reporting person is a beneficiary. The reporting person is a trustee of this trust. ( 5 )Shares are held in a trust for which the reporting person is a beneficiary. The reporting person's spouse is a trustee of this trust. ( 6 )Shares are held by the reporting person's spouse. ( 7 )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. ( 8 )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. ( 9 )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.

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

* If the form is filed by more than one reporting person, see Instruction 4(b)(v).

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