2,426,857 Boundless Bio (NASDAQ: BOLD) shares held by Zachary Hornby

Rhea-AI Filing Summary

Boundless Bio, Inc. (BOLD) discloses that Zachary Hornby has filed a Schedule 13G reporting beneficial ownership of its common stock. As of August 17, 2026, he beneficially owns 2,426,857 shares of common stock, including stock options exercisable within 60 days, representing 9.8% of the outstanding class based on 22,747,985 shares outstanding as of August 5, 2026. He reports sole voting and dispositive power over all of these shares and no shared voting or dispositive power.

Key Figures

Beneficially owned shares: 2,426,857 shares of Common Stock Ownership percentage: 9.8% Shares held directly: 456,410 shares of Common Stock +2 more

5 metrics

Beneficially owned shares 2,426,857 shares of Common Stock Beneficial ownership as of August 17, 2026

Ownership percentage 9.8% Percent of Boundless Bio common stock outstanding

Shares held directly 456,410 shares of Common Stock Portion of beneficial ownership held directly by Zachary Hornby

Shares underlying stock options 1,970,447 shares of Common Stock Underlying stock options vested or vesting within 60 days of August 17, 2026

Shares outstanding 22,747,985 shares of Common Stock Shares outstanding as of August 5, 2026, used to calculate ownership percentage

Key Terms

Schedule 13G, beneficial ownership, sole voting power, sole dispositive power, +1 more

5 terms

Schedule 13G regulatory

"This statement is filed on behalf of Zachary Hornby"

A Schedule 13G is a formal document that investors file with the government when they acquire a large ownership stake in a company, usually for investment purposes rather than control. It helps keep the public informed about who owns significant parts of a company's shares, which can influence how the company is managed and how investors make decisions. Filing this schedule is important for transparency and understanding the ownership landscape of publicly traded companies.

beneficial ownership financial

"The ownership information presented herein represents beneficial ownership"

Beneficial ownership means the person or entity that actually enjoys the benefits of owning shares or other assets — such as receiving dividends, voting rights, or price gains — even if the legal title is held in another name. For investors it matters because knowing who truly controls and profits from a company reveals who can influence decisions, exposes potential conflicts of interest or hidden concentration of power, and affects transparency and risk in the stock.

sole voting power financial

"Sole power to vote or to direct the vote: 2,426,857"

Sole voting power is the exclusive right to cast votes attached to a shareholder’s stock without needing approval from anyone else. Like holding the only remote control for a TV, it lets that holder decide corporate matters such as board members, mergers, and policy changes, making it important to investors because it concentrates control and can strongly influence a company’s strategy and the value of its shares.

sole dispositive power financial

"Sole power to dispose or to direct the disposition of: 2,426,857"

Sole dispositive power is the exclusive legal authority to decide what happens to a security — for example, whether to sell, transfer, or retain shares — without needing anyone else’s permission. Investors care because it signals who truly controls the economic outcome of an investment: like holding the only key to a safe, the holder can realize gains or losses and may trigger regulatory reporting, insider rules, or influence over corporate ownership.

stock options financial

"shares of Common Stock underlying stock options that are vested"

Stock options are agreements that give a person the right to buy or sell a company's stock at a specific price within a certain time frame. They are often used as a reward or incentive, similar to a coupon that can be used later if the stock price rises, allowing the holder to make a profit.

FAQ

How many Boundless Bio (BOLD) shares does Zachary Hornby beneficially own?

Zachary Hornby beneficially owns 2,426,857 shares of Boundless Bio common stock as of August 17, 2026, including shares underlying stock options that are vested or will vest within 60 days of that date.

What percentage of Boundless Bio (BOLD) does Zachary Hornby own according to this Schedule 13G?

According to the Schedule 13G, Zachary Hornby beneficially owns 9.8% of Boundless Bio’s outstanding common stock, based on 22,747,985 shares outstanding as of August 5, 2026.

How many Boundless Bio (BOLD) shares owned by Zachary Hornby are from stock options?

Out of the 2,426,857 shares reported, 1,970,447 shares are underlying stock options that are vested or will vest within 60 days of August 17, 2026. The remaining 456,410 shares are held directly.

Does Zachary Hornby share voting or dispositive power over his Boundless Bio (BOLD) shares?

No. The filing states that Zachary Hornby has sole voting power and sole dispositive power over 2,426,857 shares and no shared voting or dispositive power over any shares.

What share count for Boundless Bio (BOLD) is used to calculate Zachary Hornby’s ownership percentage?

The ownership percentage is calculated using 22,747,985 shares of Boundless Bio common stock outstanding as of August 5, 2026, as disclosed in the company’s Quarterly Report on Form 10-Q.

AI-generated analysis. How Rhea-AI works. Not financial advice.

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Available on EDGAR 08/28/2026 - 09:00 AM Accepted by SEC EDGAR 08/27/2026 - 09:05 PM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Boundless Bio, Inc.

(Name of Issuer)

Common Stock, par value $0.0001 per share

(Title of Class of Securities)

10170A100

(CUSIP Number)

10/14/2025

(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)

schemaVersion:

SCHEDULE 13G

CUSIP Number(s): 10170A100

1

Names of Reporting Persons

Zachary Hornby

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 STATES

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 2,426,857.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 2,426,857.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,426,857.00

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)

9.8 %

12

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Boundless Bio, Inc.
(b)	Address of issuer's principal executive offices: 11099 North Torrey Pines Road, Suite 150, La Jolla, CA 92037
Item 2.
(a)	Name of person filing: This statement is filed on behalf of Zachary Hornby (the "Reporting Person").
(b)	Address or principal business office or, if none, residence: c/o Boundless Bio, Inc., 11099 North Torrey Pines Road, Suite 150, La Jolla, CA 92037
(c)	Citizenship: The Reporting Person is a citizen of the United States.
(d)	Title of class of securities: Common Stock, par value $0.0001 per share
(e)	CUSIP Number(s): 10170A100
Item 3.	If this statement is filed pursuant to §§ 240.13d-1(b) or 240.13d-2(b) or (c), check whether the person filing is a:
(a)	Broker or dealer registered under section 15 of the Act (15 U.S.C. 78o);
(b)	Bank as defined in section 3(a)(6) of the Act (15 U.S.C. 78c);
(c)	Insurance company as defined in section 3(a)(19) of the Act (15 U.S.C. 78c);
(d)	Investment company registered under section 8 of the Investment Company Act of 1940 (15 U.S.C. 80a-8);
(e)	An investment adviser in accordance with § 240.13d-1(b)(1)(ii)(E);
(f)	An employee benefit plan or endowment fund in accordance with § 240.13d-1(b)(1)(ii)(F);
(g)	A parent holding company or control person in accordance with § 240.13d-1(b)(1)(ii)(G);
(h)	A savings associations as defined in Section 3(b) of the Federal Deposit Insurance Act (12 U.S.C. 1813);
(i)	A church plan that is excluded from the definition of an investment company under section 3(c)(14) of the Investment Company Act of 1940 (15 U.S.C. 80a-3);
(j)	A non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J). If filing as a non-U.S. institution in accordance with § 240.13d-1(b)(1)(ii)(J), please specify the type of institution:
(k)	Group, in accordance with Rule 240.13d-1(b)(1)(ii)(K).
Item 4.	Ownership
(a)	Amount beneficially owned: The ownership information presented herein represents beneficial ownership of Common Stock of the Issuer as of August 17, 2026, based upon 22,747,985 shares of Common Stock outstanding as of August 5, 2026, as disclosed in the Issuer's Quarterly Report on Form 10-Q filed with the Securities and Exchange Commission on August 7, 2026. The Reporting Person is the beneficial owner of 2,426,857 shares of Common Stock, which consist of (i) 456,410 shares of Common Stock held directly by the Reporting Person, and (ii) 1,970,447 shares of Common Stock underlying stock options that are vested or will vest within 60 days of August 17, 2026.
(b)	Percent of class: 9.8%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: 2,426,857
	(ii) Shared power to vote or to direct the vote: 0
	(iii) Sole power to dispose or to direct the disposition of: 2,426,857
	(iv) Shared power to dispose or to direct the disposition of: 0
Item 5.	Ownership of 5 Percent or Less of a Class.
	Not Applicable
Item 6.	Ownership of more than 5 Percent on Behalf of Another Person.
	Not Applicable
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

Item 10.

Certifications:

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.

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.

Zachary Hornby

Signature:	/s/ Zachary Hornby
Name/Title:	Zachary Hornby
Date:	08/27/2026