FMR LLC reports 6.2% stake in MBX Biosciences

FMR LLC reported beneficial ownership of 2,936,684 shares of MBX Biosciences Inc. common stock, representing 6.2% of the class as reported on 03/31/2026.

Rhea-AI Filing Summary

FMR LLC reported beneficial ownership of 2,936,684 shares of MBX Biosciences Inc. common stock, representing 6.2% of the class as reported on 03/31/2026. The filing lists sole dispositive power for 2,936,684 shares and includes Abigail P. Johnson as having shared dispositive power via FMR LLC.

Insights

Institutional Ownership Analyst

Fidelity-affiliated manager reports a sizeable passive stake in MBX.

FMR LLC discloses beneficial ownership of 2,936,684 shares representing 6.2% of MBX common stock as of 03/31/2026. The filing shows sole dispositive power and identifies Abigail P. Johnson in delegation roles.

Ownership at this scale is a notable passive stake; timing and any trading intent are not stated. Subsequent filings could clarify whether this is passive investment reporting under Schedule 13G or if positions change.

Compliance & Governance Specialist

Disclosure aligns with Schedule 13G reporting conventions for institutional investors.

The schedule identifies FMR LLC as the reporting person, provides address details, cites CUSIP 55287L101, and attaches an exhibit for a 13d-1(k) agreement. Signatures reference a power of attorney effective April 13, 2026.

Corporate governance teams should track any future amendments or switches to Schedule 13D that would indicate activist intent; this filing itself is informational.

Key Figures

Beneficial ownership: 2,936,684 shares Percent of class: 6.2% CUSIP: 55287L101 +3 more

6 metrics

Beneficial ownership 2,936,684 shares Amount reported as beneficially owned by FMR LLC (as of 03/31/2026)

Percent of class 6.2% Percent of MBX common stock represented by reported holdings

CUSIP 55287L101 Identifier for MBX Biosciences common stock

Reporting date 03/31/2026 Date to which the ownership amount is tied

Power of attorney effective 04/13/2026 Effective date referenced for the power of attorney in signatures

Signature date 05/05/2026 Date signatures were executed on the filing

Key Terms

Schedule 13G, Beneficial ownership, Sole Dispositive Power, 13d-1(k) agreement, +1 more

5 terms

Schedule 13G regulatory

"Item 1. (a) Name of issuer: MBX BIOSCIENCES INC"

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

"Item 4. Ownership (a) Amount beneficially owned: 2936684.00"

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 Dispositive Power regulatory

"Item 4. (iii) Sole power to dispose or to direct the disposition of: 2936684.00"

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.

13d-1(k) agreement regulatory

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

Power of Attorney legal

"Duly authorized under Power of Attorney effective as of April 13, 2026"

A power of attorney is a legal document that allows one person to make decisions and act on behalf of another person, often in financial or legal matters. It’s like giving someone a trusted helper or agent the authority to handle important tasks if you are unable to do so yourself. This matters to investors because it can impact how their assets are managed or transferred if they become unable to oversee their affairs.

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 2,936,684 shares, equal to 6.2% of MBX common stock as of 03/31/2026. The schedule records sole dispositive power for the same share amount and cites CUSIP 55287L101.

Who is listed as having voting or dispositive power over the MBX shares?

The filing lists FMR LLC as holding sole voting and sole dispositive power over the identified shares, and shows Abigail P. Johnson associated with shared dispositive power via FMR LLC in the cover responses.

What form was filed to report FMR LLC's stake in MBX?

The reporting vehicle is a Schedule 13G filing for passive institutional ownership, dated with holdings as of 03/31/2026, and includes an exhibit referencing a 13d-1(k) agreement and a power of attorney.

Does this filing indicate activist intentions or planned sales?

This Schedule 13G discloses beneficial ownership and control powers but does not state any activist intentions or planned transactions. It provides signature and exhibit references; transaction intent is not described in the excerpt.

What identifying details are included for MBX shares in the filing?

The filing provides the class as Common Stock and the CUSIP number 55287L101. It also lists the issuer address at 11711 N. Meridian Street, Suite 300, Carmel, IN and the reporting person’s address at 245 Summer Street, Boston, MA.

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

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Accepted by SEC EDGAR 05/06/2026 - 11:30 AM Learn about SEC filing dates

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

MBX BIOSCIENCES INC

(Name of Issuer)

COMMON STOCK

(Title of Class of Securities)

55287L101

(CUSIP Number)

03/31/2026

(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 2,933,744.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 2,936,684.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,936,684.00

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)

6.2 %

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 2,936,684.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

2,936,684.00

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)

6.2 %

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 Number(s): 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: 2936684.00
(b)	Percent of class: 6.2 %
(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: 2936684.00
	(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:	05/05/2026

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:	05/05/2026

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.