PRIMECAP discloses 5.21% Rhythm Pharmaceuticals (RYTM) ownership in 13G/A

Rhea-AI Filing Summary

PRIMECAP Management Co. filed an amended Schedule 13G reporting its beneficial ownership of common stock of RYTM Pharmaceuticals, Inc. As of the event date, PRIMECAP reports beneficial ownership of 3,478,669 shares, representing 5.21% of the company’s outstanding class.

PRIMECAP has sole power to vote and dispose of all 3,478,669 shares, with no shared voting or dispositive power. The firm certifies the securities were acquired and are held in the ordinary course of business and not for the purpose of changing or influencing control of Rhythm Pharmaceuticals.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 12)

RHYTHM PHARMACEUTICALS, INC.

(Name of Issuer)

COM

(Title of Class of Securities)

76243J105

(CUSIP Number)

12/31/2025

(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)

SCHEDULE 13G

CUSIP No. 76243J105

1

Names of Reporting Persons

PRIMECAP MANAGEMENT CO/CA/

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 3,478,669.00
6	Shared Voting Power 0.00
7	Sole Dispositive Power 3,478,669.00
8	Shared Dispositive Power 0.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

3,478,669.00

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)

5.21 %

12

Type of Reporting Person (See Instructions)

IA

SCHEDULE 13G

Item 1.
(a)	Name of issuer: RHYTHM PHARMACEUTICALS, INC.
(b)	Address of issuer's principal executive offices: 222 BERKELEY STREET, 12TH FLOOR, BOSTON, MASSACHUSETTS 02116
Item 2.
(a)	Name of person filing: PRIMECAP MANAGEMENT CO/CA/
(b)	Address or principal business office or, if none, residence: 177 EAST COLORADO BLVD. 11TH FLOOR PASADENA, California 91105
(c)	Citizenship: PRIMECAP MANAGEMENT CO/CA/ - UNITED STATES
(d)	Title of class of securities: COM
(e)	CUSIP No.: 76243J105
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: 3,478,669
(b)	Percent of class: 5.21 %
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: PRIMECAP MANAGEMENT CO/CA/ - 3,478,669
	(ii) Shared power to vote or to direct the vote: PRIMECAP MANAGEMENT CO/CA/ - 0
	(iii) Sole power to dispose or to direct the disposition of: PRIMECAP MANAGEMENT CO/CA/ - 3,478,669
	(iv) Shared power to dispose or to direct the disposition of: PRIMECAP MANAGEMENT CO/CA/ - 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 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.

PRIMECAP MANAGEMENT CO/CA/

Signature:	Jorge A. Rodriguez
Name/Title:	Deputy Chief Compliance Officer
Date:	02/11/2026

FAQ

What stake does PRIMECAP Management Co. report in RYTM Pharmaceuticals?

PRIMECAP Management Co. reports beneficial ownership of 3,478,669 shares of Rhythm Pharmaceuticals, representing 5.21% of the outstanding class. This stake gives PRIMECAP a significant institutional position while remaining a passive holder under Schedule 13G criteria.

Does PRIMECAP have voting control over its RYTM shares?

Yes. PRIMECAP reports sole voting power over 3,478,669 Rhythm Pharmaceuticals shares and no shared voting power. It also reports sole dispositive power over the same number of shares, indicating it alone decides how these shares are voted and when they may be sold.

Is PRIMECAP’s RYTM position intended to influence control of the company?

No. PRIMECAP certifies the Rhythm Pharmaceuticals shares were acquired and are held in the ordinary course of business, not to change or influence control, and not in connection with any transaction having that purpose, other than activities solely related to a nomination under Rule 240.14a-11.

What type of SEC filing did PRIMECAP submit for RYTM Pharmaceuticals?

PRIMECAP submitted an Amendment No. 12 to Schedule 13G for Rhythm Pharmaceuticals. Schedule 13G is used by certain passive or institutional investors to report beneficial ownership of more than 5% of a class of a company’s registered equity securities.

How is PRIMECAP classified in the RYTM ownership filing?

The filing identifies PRIMECAP Management Co. as a United States entity and provides a checkbox list for classifications such as investment adviser and other institutional categories under Rule 13d-1(b), though the excerpt does not indicate a checked category within that list.

When was the RYTM ownership level triggering this Schedule 13G/A measured?

The ownership level was measured as of December 31, 2025, which is listed as the date of the event requiring the filing. This date anchors the reported 3,478,669 shares and the corresponding 5.21% beneficial ownership in Rhythm Pharmaceuticals.