EcoR1 holds 9.4% stake in Mereo BioPharma (MREO) via ADSs

Rhea-AI Filing Summary

Mereo BioPharma Group plc received a Schedule 13G filing showing that funds associated with EcoR1 Capital and its principal Oleg Nodelman beneficially own 74,500,000 Ordinary Shares, equal to 9.4% of the outstanding Ordinary Shares. These holdings are held through 14,900,000 American Depositary Shares, each ADS representing 5 Ordinary Shares.

Within this total, EcoR1 Capital Fund Qualified, L.P. is reported as beneficially owning 69,802,840 Ordinary Shares, or 8.8% of the class, through 13,960,568 ADSs. The filing states that the securities were acquired and are held in the ordinary course of business and not for the purpose of changing or influencing control of the company. The reporting persons also formally disclaim group status and beneficial ownership beyond their pecuniary interest.

Insights

Institutional ownership analyst

EcoR1 reports a sizable but passive 9.4% stake in Mereo BioPharma.

The filing shows EcoR1 Capital, its Qualified Fund, and Oleg Nodelman jointly reporting beneficial ownership of 74,500,000 Ordinary Shares of Mereo BioPharma Group plc. This represents 9.4% of the company’s Ordinary Shares outstanding as of November 7, 2025, calculated against 795,658,504 Ordinary Shares. The position is held via 14,900,000 American Depositary Shares, each representing 5 Ordinary Shares.

Within this total, EcoR1 Capital Fund Qualified, L.P. accounts for 69,802,840 Ordinary Shares, or 8.8% of the class, through 13,960,568 ADSs. The filing specifies that the shares are held in the ordinary course of business and not with the purpose or effect of changing or influencing control, consistent with a passive Schedule 13G status. EcoR1 is identified as general partner and investment adviser to its funds, and Nodelman as the control person of EcoR1, with each party disclaiming beneficial ownership beyond its pecuniary interest.

The disclosure formalizes EcoR1’s role as a significant institutional holder without signaling an activist or control-seeking stance. Future ownership levels and any change from a passive to a more active posture, if they occur, would typically be reflected in later ownership reports or a shift from Schedule 13G to another filing type.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

Mereo BioPharma Group plc

(Name of Issuer)

Ordinary Shares

(Title of Class of Securities)

589492107

(CUSIP Number)

01/02/2026

(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)

SCHEDULE 13G

CUSIP No. 589492107

1

Names of Reporting Persons

EcoR1 Capital, 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 0.00
6	Shared Voting Power 74,500,000.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 74,500,000.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

74,500,000.00

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)

9.4 %

12

Type of Reporting Person (See Instructions)

IA, OO

Comment for Type of Reporting Person: These Ordinary Shares are held through 14,900,000 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 795,658,504 Ordinary Shares outstanding on November 7, 2025, as reported in the Form 10-Q filed by the Issuer for the quarter ended September 30, 2025.

SCHEDULE 13G

CUSIP No. 589492107

1

Names of Reporting Persons

Oleg Nodelman

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 74,500,000.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 74,500,000.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

74,500,000.00

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)

9.4 %

12

Type of Reporting Person (See Instructions)

HC, IN

Comment for Type of Reporting Person: These Ordinary Shares are held through 14,900,000 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 795,658,504 Ordinary Shares outstanding on November 7, 2025, as reported in the Form 10-Q filed by the Issuer for the quarter ended September 30, 2025.

SCHEDULE 13G

CUSIP No. 589492107

1

Names of Reporting Persons

EcoR1 Capital Fund Qualified, L.P.

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 0.00
6	Shared Voting Power 69,802,840.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 69,802,840.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

69,802,840.00

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)

8.8 %

12

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person: These Ordinary Shares are held through 13,960,568 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 795,658,504 Ordinary Shares outstanding on November 7, 2025, as reported in the Form 10-Q filed by the Issuer for the quarter ended September 30, 2025.

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Mereo BioPharma Group plc
(b)	Address of issuer's principal executive offices: ONE CAVENDISH PLACE, FOURTH FLOOR, LONDON, United Kingdom, W1G 0QF
Item 2.
(a)	Name of person filing: EcoR1 Capital, LLC, a Delaware limited liability company ("EcoR1") EcoR1 Capital Fund Qualified, L.P., a Delaware limited partnership ("Qualified Fund") Oleg Nodelman Qualified Fund is filing this statement jointly with the other reporting persons, but not as a member of a group, and it expressly disclaims membership in a group. In addition, the filing of this Schedule 13G on behalf of Qualified Fund should not be construed as an admission that it is, and it disclaims that it is, a beneficial owner, as defined in Rule 13d-3 under the Act, of any securities covered by this Schedule 13G. Each reporting person also disclaims beneficial ownership of the securities reported herein except to the extent of that person's pecuniary interest therein.
(b)	Address or principal business office or, if none, residence: 357 Tehama Street #3 San Francisco, CA 94103
(c)	Citizenship: See Item 4 of the cover sheet for each reporting person.
(d)	Title of class of securities: Ordinary Shares
(e)	CUSIP No.: 589492107
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: EcoR1: 74,500,000 Oleg Nodelman: 74,500,000 Qualified Fund: 69,802,840
(b)	Percent of class: EcoR1: 9.4% Oleg Nodelman: 9.4% Qualified Fund: 8.8%
(c)	Number of shares as to which the person has:
	(i) Sole power to vote or to direct the vote: EcoR1: 0 Oleg Nodelman: 0 Qualified Fund: 0
	(ii) Shared power to vote or to direct the vote: EcoR1: 74,500,000 Oleg Nodelman: 74,500,000 Qualified Fund: 69,802,840
	(iii) Sole power to dispose or to direct the disposition of: EcoR1: 0 Oleg Nodelman: 0 Qualified Fund: 0
	(iv) Shared power to dispose or to direct the disposition of: EcoR1: 74,500,000 Oleg Nodelman: 74,500,000 Qualified Fund: 69,802,840
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.
	If a group has filed this schedule pursuant to §240.13d-1(b)(1)(ii)(K), so indicate under Item 3(k) and attach an exhibit stating the identity and Item 3 classification of each member of the group. If a group has filed this schedule pursuant to §240.13d-1(c) or §240.13d-1(d), attach an exhibit stating the identity of each member of the group. EcoR1 is the general partner and investment adviser of investment funds, including Qualified Fund. Mr. Nodelman is the control person of EcoR1.
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. 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.

EcoR1 Capital, LLC

Signature:	/s/ Oleg Nodelman
Name/Title:	Manager
Date:	01/09/2026

Oleg Nodelman

Signature:	/s/ Oleg Nodelman
Name/Title:	Reporting person
Date:	01/09/2026

EcoR1 Capital Fund Qualified, L.P.

Signature:	/s/ Oleg Nodelman
Name/Title:	Manager of the General Partner, EcoR1 Capital, LLC
Date:	01/09/2026

Exhibit Information

EXHIBIT 99 - AGREEMENT REGARDING JOINT FILING OF STATEMENT ON SCHEDULE 13D OR 13G

FAQ

How many Mereo BioPharma (MREO) shares does EcoR1 Capital report owning?

EcoR1 Capital, together with related reporting persons, reports beneficial ownership of 74,500,000 Ordinary Shares of Mereo BioPharma Group plc. These are held through 14,900,000 American Depositary Shares, with each ADS representing 5 Ordinary Shares.

What percentage of Mereo BioPharmas Ordinary Shares does EcoR1 Capital control under this Schedule 13G?

The filing states that EcoR1 Capital, Oleg Nodelman, and related entities beneficially own 9.4% of the outstanding Ordinary Shares of Mereo BioPharma Group plc. This percentage is calculated based on 795,658,504 Ordinary Shares outstanding as of November 7, 2025.

How much of Mereo BioPharma (MREO) is held by EcoR1 Capital Fund Qualified, L.P.?

EcoR1 Capital Fund Qualified, L.P. is reported as beneficially owning 69,802,840 Ordinary Shares of Mereo BioPharma, representing 8.8% of the Ordinary Shares outstanding. These are held through 13,960,568 ADSs, each ADS representing 5 Ordinary Shares.

Are EcoR1 Capital and Oleg Nodelman filing as active or passive investors in Mereo BioPharma?

They filed a Schedule 13G and certify that the securities were acquired and are held in the ordinary course of business, not for the purpose or with the effect of changing or influencing control of Mereo BioPharma. This is consistent with a passive ownership stance.

How are the Mereo BioPharma holdings structured between ADSs and Ordinary Shares?

The holdings are held via American Depositary Shares (ADSs). For the 9.4% position, EcoR1-related entities hold 14,900,000 ADSs, and each ADS represents 5 Ordinary Shares, resulting in 74,500,000 Ordinary Shares beneficially owned.

Do the EcoR1 reporting persons claim to be a group or controlling shareholders of Mereo BioPharma?

The filing states that Qualified Fund is filing jointly with the other reporting persons but expressly disclaims membership in a group. Each reporting person also disclaims beneficial ownership of the reported securities except to the extent of that persons pecuniary interest, and they certify the holdings are not for changing or influencing control.