Sec Form 4 Filing - Burow Kristina @ Boundless Bio, Inc. - 2024-04-02

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 Burow Kristina 2. Issuer Name and Ticker or Trading Symbol Boundless Bio, Inc. [ BOLD] 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 BOUNDLESS BIO, INC., 9880 CAMPUS POINT DRIVE, SUITE 120 3. Date of Earliest Transaction (MM/DD/YY) 04/02/2024 (Street) SAN DIEGO, CA92121 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 04/02/2024 C 589,743 A 617,092 I See footnotes ( 2 ) ( 4 ) ( 5 ) Common Stock 04/02/2024 C 787,545 A 828,570 I See footnotes ( 3 ) ( 4 ) ( 5 ) Common Stock 04/02/2024 C 1,048,433 A 1,048,433 I See footnotes ( 6 ) ( 7 ) ( 8 ) Common Stock 04/02/2024 P 66,667 A $ 16 683,759 I See footnotes ( 2 ) ( 4 ) ( 5 ) ( 9 ) Common Stock 04/02/2024 P 133,333 A $ 16 1,181,766 I See footnotes ( 6 ) ( 7 ) ( 8 ) ( 10 )

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 Convertible Preferred Stock ( 1 ) 04/02/2024 C 424,908 ( 1 ) ( 1 ) Common Stock 424,908 ( 1 ) 0 I See footnotes ( 2 ) ( 4 ) ( 5 ) Series A Convertible Preferred Stock ( 1 ) 04/02/2024 C 586,080 ( 1 ) ( 1 ) Common Stock 586,080 ( 1 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series A Convertible Preferred Stock ( 1 ) 04/02/2024 C 454,212 ( 1 ) ( 1 ) Common Stock 454,212 ( 1 ) 0 I See footnotes ( 6 ) ( 7 ) ( 8 ) Series B Convertible Preferred Stock ( 1 ) 04/02/2024 C 227,920 ( 1 ) ( 1 ) Common Stock 227,920 ( 1 ) 0 I See footnotes ( 6 ) ( 7 ) ( 8 ) Series C Convertible Preferred Stock ( 1 ) 04/02/2024 C 164,835 ( 1 ) ( 1 ) Common Stock 164,835 ( 1 ) 0 I See footnotes ( 2 ) ( 4 ) ( 5 ) Series C Convertible Preferred Stock ( 1 ) 04/02/2024 C 201,465 ( 1 ) ( 1 ) Common Stock 201,465 ( 1 ) 0 I See footnotes ( 3 ) ( 4 ) ( 5 ) Series C Convertible Preferred Stock ( 1 ) 04/02/2024 C 366,301 ( 1 ) ( 1 ) Common Stock 366,301 ( 1 ) 0 I See footnotes ( 6 ) ( 7 ) ( 8 )

Reporting Owners Reporting Owner Name / Address Relationships Director 10% Owner Officer Other Burow Kristina C/O BOUNDLESS BIO, INC. 9880 CAMPUS POINT DRIVE, SUITE 120 SAN DIEGO, CA92121 X X Signatures /s/ Jessica Oien, Attorney-in-Fact for Kristina Burow 04/04/2024 Signature of Reporting Person Date

Explanation of Responses:

( 1 )Each share of the Issuer's preferred stock is convertible into shares of the Issuer's common stock and has no expiration date. The preferred stock automatically converted into common stock upon the closing of the Issuer's initial public offering. ( 2 )These shares are held directly by ARCH Venture Fund IX, L.P. (ARCH IX). ARCH Venture Partners IX, L.P. (AVP IX LP) is the sole general partner of ARCH IX. ( 3 )These shares are held directly by ARCH Venture Fund IX Overage, L.P. (ARCH IX Overage). ARCH Venture Partners IX Overage, L.P. (AVP IX Overage LP) is the sole general partner of ARCH IX Overage. ( 4 )ARCH Venture Partners IX, LLC (AVP IX LLC) is the sole general partner of each of AVP IX LP and AVP IX Overage LP. Keith Crandell, Robert Nelsen and Clint Bybee are managing directors of AVP IX LLC ("AVP IX MDs"). AVP IX LP and AVP IX Overage LP may be deemed to beneficially own the shares held by ARCH IX and ARCH IX Overage, respectively, AVP IX LLC may be deemed to beneficially own the shares held by ARCH IX and ARCH IX Overage, and the AVP IX MDs may be deemed to share the power to direct the disposition and vote of the shares held by ARCH IX and ARCH IX Overage. AVP IX LP, AVP IX Overage LP, AVP IX LLC and the AVP IX MDs have filed a separate Form 4 and disclaim beneficial ownership except to the extent of any pecuniary interest therein, if any. ( 5 )The Reporting Person has an interest in the AVP IX LP and AVFP IX Overage LP but does not have voting or investment control over the shares held by ARCH IX and ARCH IX Overage. The Reporting Person disclaims ownership of such shares, except to the extent of her pecuniary interest therein. ( 6 )These shares are held directly by ARCH Venture Fund X Overage, L.P. (ARCH X Overage). ARCH Venture Partners X Overage, L.P. (AVP X Overage LP) is the sole general partner of ARCH X Overage. ( 7 )ARCH Venture Partners X, LLC (AVP X LLC) is the sole general partner of AVP X Overage LP. In addition to the Reporting Person, Keith Crandell, Robert Nelsen and Steven Gillis are members of the investment committee of AVP X LLC (the "AVP X Committee Members"). AVP X Overage LP may be deemed to beneficially own the shares held by ARCH X Overage, AVP X LLC may be deemed to beneficially own the shares held by ARCH X Overage, and each of the AVP X Committee Members may be deemed to share the power to direct the disposition and vote of the shares held by ARCH X Overage. AVP X Overage LP, AVP X LLC, and the AVP X Committee Members other than the Reprting Person have filed a separate Form 4 and disclaim beneficial ownership except to the extent of any pecuniary interest therein, if any. ( 8 )The Reporting Person is also an AVP X Committee Member and may be deemed to share the power to direct the disposition and vote of the shares held by ARCH X Overage. The Reporting Person disclaims beneficial ownership of such shares except to any pecuniary interest therein, and this report shall not be deemed an admission that such shares are beneficially owned by her for Section 16 or any other purpose. ( 9 )Reflects shares purchased by ARCH IX in the Issuer's initial public offering. ( 10 )Reflects shares purchased by ARCH X Overage in the Issuer's initial public offering.

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).

** Intentional misstatements or omissions of facts constitute Federal Criminal Violations. See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).

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