FMR LLC reports 11.2% stake in MBX Biosciences

MBX BIOSCIENCES INC filed an Amendment (Schedule 13G/A) disclosing that FMR LLC beneficially owns 5,322,884.68 shares of common stock, representing 11.2% of the class as of 06/30/2026.

Rhea-AI Filing Summary

MBX BIOSCIENCES INC filed an Amendment (Schedule 13G/A) disclosing that FMR LLC beneficially owns 5,322,884.68 shares of common stock, representing 11.2% of the class as of 06/30/2026. The filing lists Abigail P. Johnson as having dispositive power over the same 5,322,884.68 shares. The report is signed under a power of attorney and references an attached 13d-1(k)(1) agreement in Exhibit 99.

Insights

Institutional Ownership Analyst

FMR LLC holds an 11.2% stake via dispositive authority.

FMR LLC is shown as beneficial owner of 5,322,884.68 shares representing 11.2% of MBX Biosciences' common stock as of 06/30/2026. The filing attributes sole dispositive power to FMR LLC and cites Abigail P. Johnson as having dispositive authority for the same share count.

Shareholders should note that the filing is an amended Schedule 13G/A and includes an executed power of attorney; cash‑flow treatment and any planned transactions by the holder are not stated in the excerpt.

Corporate Governance Expert

Amendment documents authority and an exhibit agreement; no transaction timing disclosed.

The amendment references an attached Exhibit 99 and a power of attorney effective 04/13/2026 incorporated by reference. The signature block shows authorization by Richard Bourgelas on behalf of FMR LLC and Abigail P. Johnson.

Filing text specifies that one or more other persons may have rights to dividends or sale proceeds but that no other person holds >5% individually. Further disclosures about underlying funds, voting arrangements, or subsequent trades are not included in this excerpt.

Key Figures

Beneficial ownership: 5,322,884.68 shares Percent of class: 11.2% Dispositive power: 5,322,884.68 shares +2 more

5 metrics

Beneficial ownership 5,322,884.68 shares Amount reported as beneficially owned by FMR LLC (Item 4)

Percent of class 11.2% Percent of common stock represented by reported shares as of 06/30/2026

Dispositive power 5,322,884.68 shares Sole power to dispose as reported in Item 4

Power of attorney effective 04/13/2026 Date of power of attorney referenced in signature block

Signature date 07/07/2026 Date signatures were executed on the amendment

Key Terms

Schedule 13G/A, beneficially owned, dispositive power, 13d-1(k)(1) agreement

4 terms

Schedule 13G/A regulatory

"Amendment (Schedule 13G/A) disclosing beneficial ownership"

A Schedule 13G/A is an amended public filing with the U.S. securities regulator that updates a previous Schedule 13G, disclosing when an individual or group holds a substantial (typically over 5%) stake in a company and is claiming a passive, non‑controlling intent. Investors monitor these updates because rising or falling holdings can signal changing confidence, potential future moves, or shifts in voting power — like watching a public ledger where large shareholders quietly adjust their positions.

beneficially owned financial

"Amount beneficially owned: 5322884.68"

Beneficially owned describes securities or assets where a person has the economic rights and control—such as the right to receive dividends and to direct voting—even if legal title is held in another name. Think of it like having the keys and using a car that’s registered to someone else: you get the benefits and make decisions. Investors care because beneficial ownership reveals who truly controls value and voting power, affecting corporate decisions and takeover dynamics.

dispositive power regulatory

"Sole power to dispose or to direct the disposition of: 5322884.68"

Dispositive power is the authority to decide the final outcome of an asset, legal claim, contract, or corporate action — in effect the power to dispose of or resolve something. For investors it matters because whoever holds that authority can determine who gets paid, who controls an asset or vote, and how risks and returns are allocated; think of it like holding the key that lets you lock in the winner or loser in a deal.

13d-1(k)(1) agreement regulatory

"Please see Exhibit 99 for 13d-1(k) (1) agreement"

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What stake does FMR LLC report in MBX Biosciences (MBX)?

FMR LLC reports beneficial ownership of 5,322,884.68 shares, equal to 11.2% of the common stock as of 06/30/2026. The filing shows sole dispositive power over those shares.

Does Abigail P. Johnson appear in the MBX 13G/A filing?

Yes. Abigail P. Johnson is listed with dispositive authority over 5,322,884.68 shares, matching the 11.2% position reported by FMR LLC in the amendment.

Is the filing an amendment and does it reference supporting exhibits?

The document is titled an amendment (Schedule 13G/A) and references an attached Exhibit 99 for a 13d-1(k)(1) agreement and a power of attorney incorporated by reference.

Does the filing identify other parties with >5% ownership?

The amendment states some other persons have rights to dividends or sale proceeds, but explicitly says no other person's interest exceeds 5% of the outstanding common stock.

Who signed the MBX Schedule 13G/A amendment?

The signatures show Richard Bourgelas signing as duly authorized under a power of attorney on behalf of FMR LLC and on behalf of Abigail P. Johnson, dated 07/07/2026.

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

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Accepted by SEC EDGAR 07/08/2026 - 10:11 AM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 1)

MBX BIOSCIENCES INC

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

55287L101

(CUSIP Number)

06/30/2026

(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): 55287L101

1

Names of Reporting Persons

FMR LLC

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

DELAWARE

Number of Shares Beneficially Owned by Each Reporting Person With:

5	Sole Voting Power 5,322,706.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 5,322,884.68
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

5,322,884.68

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)

11.2 %

12

Type of Reporting Person (See Instructions)

HC

SCHEDULE 13G

CUSIP Number(s): 55287L101

1

Names of Reporting Persons

Abigail P. Johnson

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 0.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 5,322,884.68
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

5,322,884.68

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)

11.2 %

12

Type of Reporting Person (See Instructions)

IN

SCHEDULE 13G

Item 1.
(a)	Name of issuer: MBX BIOSCIENCES INC
(b)	Address of issuer's principal executive offices: 11711 N. MERIDIAN STREET,SUITE 300,CARMEL,IN,USA,46032
Item 2.
(a)	Name of person filing: FMR LLC
(b)	Address or principal business office or, if none, residence: 245 Summer Street, Boston, Massachusetts 02210
(c)	Citizenship: Not applicable
(d)	Title of class of securities: COMMON STOCK
(e)	CUSIP No.: 55287L101
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: 5322884.68
(b)	Percent of class: 11.2 %
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: Please see the responses to Items 5 and 6 on the cover page.
	(ii) Shared power to vote or to direct the vote: 0.00
	(iii) Sole power to dispose or to direct the disposition of: 5322884.68
	(iv) Shared power to dispose or to direct the disposition of: 0.00
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.
	If any other person is known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, such securities, a statement to that effect should be included in response to this item and, if such interest relates to more than 5 percent of the class, such person should be identified. A listing of the shareholders of an investment company registered under the Investment Company Act of 1940 or the beneficiaries of employee benefit plan, pension fund or endowment fund is not required. One or more other persons are known to have the right to receive or the power to direct the receipt of dividends from, or the proceeds from the sale of, the COMMON STOCK of MBX BIOSCIENCES INC. No one other person's interest in the COMMON STOCK of MBX BIOSCIENCES INC is more than five percent of the total outstanding COMMON STOCK.
Item 7.	Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on by the Parent Holding Company or Control Person.
	If a parent holding company has filed this schedule, pursuant to Rule 13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the identity and the Item 3 classification of the relevant subsidiary. If a parent holding company has filed this schedule pursuant to Rule 13d-1(c) or Rule 13d-1(d), attach an exhibit stating the identification of the relevant subsidiary. See attached Exhibit 99.
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 acquired and are held in the ordinary course of business and 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.

FMR LLC

Signature:	Richard Bourgelas
Name/Title:	Duly authorized under Power of Attorney effective as of April 13, 2026, by and on behalf of FMR LLC and its direct and indirect subsidiaries*
Date:	07/07/2026

Abigail P. Johnson

Signature:	Richard Bourgelas
Name/Title:	Duly authorized under Power of Attorney effective as of April 13, 2026, by and on behalf of Abigail P. Johnson*
Date:	07/07/2026

Comments accompanying signature: *This power of attorney is incorporated herein by reference to Exhibit 24 to the Schedule 13G filed by FMR LLC on April 29,2026, accession number: 0000315066-26-000738.

Exhibit Information

Please see Exhibit 99 for 13d-1(k) (1) agreement.