EcoR1, Oleg Nodelman report 8.4% stake in Mereo (MREO)

Rhea-AI Filing Summary

Mereo BioPharma Group plc ownership disclosure: EcoR1 Capital, LLC, EcoR1 Capital Fund Qualified, L.P. and Oleg Nodelman jointly report beneficial ownership on an amendment to Schedule 13G/A. The filing shows EcoR1 and Mr. Nodelman each report shared voting and dispositive power over 66,844,275 Ordinary Shares (representing 8.4% of the class). The Qualified Fund reports shared voting and dispositive power over 62,644,610 Ordinary Shares (representing 7.8%).

The filing states these Ordinary Shares are held through 13,368,855 American Depositary Shares (each ADS = 5 Ordinary Shares) and that the percentage calculations use 798,078,829 Ordinary Shares outstanding as of March 18, 2026 as reported in the issuer's Form 10-K for the fiscal year ended December 31, 2025. The reporting persons disclaim group membership and assert the holdings were not acquired to change control.

Insights

Institutional Ownership Analyst

EcoR1 reports a single-party shared stake of 8.4% in Mereo (examples shown).

EcoR1 Capital and related reporting persons disclose shared voting and dispositive power over 66,844,275 Ordinary Shares, labeled as held through 13,368,855 ADS (ADS ratio: 1 ADS = 5 Ordinary Shares). The filing ties the percentage to March 18, 2026 outstanding shares.

Cash‑flow treatment and any planned dispositions are not stated in the excerpt; subsequent filings would show transactional activity if and when it occurs.

Governance Counsel

Filing emphasizes disclaimers and non-control intent while identifying EcoR1's advisory/GP role.

The schedule notes EcoR1 is general partner and adviser to funds including the Qualified Fund and that Mr. Nodelman is EcoR1's control person. Signatures certify the securities "were not acquired and are not held for the purpose of changing or influencing control."

Material governance implications depend on future actions; the disclosure documents current holdings and voting/dispositive power but includes an express disclaimer regarding group membership.

Key Figures

Shared voting power: 66,844,275 shares Qualified Fund shared voting power: 62,644,610 shares Ownership percent (EcoR1/Mr. Nodelman): 8.4% +3 more

6 metrics

Shared voting power 66,844,275 shares EcoR1 and Oleg Nodelman holdings reported

Qualified Fund shared voting power 62,644,610 shares EcoR1 Capital Fund Qualified, L.P. holdings reported

Ownership percent (EcoR1/Mr. Nodelman) 8.4% Percent of class using outstanding shares as of March 18, 2026

Ownership percent (Qualified Fund) 7.8% Percent of class using outstanding shares as of March 18, 2026

Ordinary Shares outstanding (used) 798,078,829 shares Outstanding as of March 18, 2026 per issuer's Form 10-K

American Depositary Shares 13,368,855 ADS ADS through which EcoR1 holdings are reported (1 ADS = 5 Ordinary Shares)

Key Terms

American Depositary Shares (ADS), Shared dispositive power, Schedule 13G/A

3 terms

American Depositary Shares (ADS) market

"These Ordinary Shares are held through 13,368,855 American Depositary Shares"

American depositary shares (ADS) are a way for investors in the United States to buy shares of foreign companies without dealing with the complexities of international markets. They represent ownership in a foreign company's stock and are traded on U.S. exchanges, making it easier and more convenient for Americans to invest internationally. ADSs allow investors to diversify their portfolios with foreign companies while using familiar trading platforms.

Shared dispositive power regulatory

"Shared Dispositive Power 66,844,275.00"

Schedule 13G/A regulatory

"Amendment No. 1 ) Mereo BioPharma Group plc ... SCHEDULE 13D OR 13G"

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.

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SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 1)

Mereo BioPharma Group plc

(Name of Issuer)

Ordinary Shares

(Title of Class of Securities)

589492107

(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): 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 66,844,275.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 66,844,275.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

66,844,275.00

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)

8.4 %

12

Type of Reporting Person (See Instructions)

IA, OO

Comment for Type of Reporting Person: These Ordinary Shares are held through 13,368,855 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 798,078,829 Ordinary Shares outstanding on March 18, 2026, as reported in the Form 10-K filed by the Issuer for the fiscal year ended December 31, 2025.

SCHEDULE 13G

CUSIP Number(s): 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 66,844,275.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 66,844,275.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

66,844,275.00

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)

8.4 %

12

Type of Reporting Person (See Instructions)

HC, IN

Comment for Type of Reporting Person: These Ordinary Shares are held through 13,368,855 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 798,078,829 Ordinary Shares outstanding on March 18, 2026, as reported in the Form 10-K filed by the Issuer for the fiscal year ended December 31, 2025.

SCHEDULE 13G

CUSIP Number(s): 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 62,644,610.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 62,644,610.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

62,644,610.00

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)

7.8 %

12

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person: These Ordinary Shares are held through 12,528,922 American Depositary Shares of the Issuer, each representing 5 Ordinary Shares. Percentage calculated based on 798,078,829 Ordinary Shares outstanding on March 18, 2026, as reported in the Form 10-K filed by the Issuer for the fiscal year ended December 31, 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: 66,844,275 Oleg Nodelman: 66,844,275 Qualified Fund: 62,644,610
(b)	Percent of class: EcoR1: 8.4% Oleg Nodelman: 8.4% Qualified Fund: 7.8%
(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: 66,844,275 Oleg Nodelman: 66,844,275 Qualified Fund: 62,644,610
	(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: 66,844,275 Oleg Nodelman: 66,844,275 Qualified Fund: 62,644,610
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)(J), so indicate under Item 3(j) 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:	05/14/2026

Oleg Nodelman

Signature:	/s/ Oleg Nodelman
Name/Title:	Reporting person
Date:	05/14/2026

EcoR1 Capital Fund Qualified, L.P.

Signature:	/s/ Oleg Nodelman
Name/Title:	Manager of the General Partner, EcoR1 Capital, LLC
Date:	05/14/2026

Exhibit Information

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

FAQ

What stake in Mereo BioPharma (MREO) does EcoR1 report?

EcoR1 reports shared voting and dispositive power over 66,844,275 Ordinary Shares, representing 8.4% of the class based on the stated outstanding share count.

How did the filing calculate the ownership percentage for MREO?

Percentages are calculated using 798,078,829 Ordinary Shares outstanding as of March 18, 2026, cited from the issuer's Form 10-K for the year ended December 31, 2025.

Are the Mereo shares held directly or through ADSs?

The filing states the Ordinary Shares are held through 13,368,855 American Depositary Shares, with each ADS representing 5 Ordinary Shares as disclosed in the excerpt.

Does the Schedule 13G/A say EcoR1 seeks control of Mereo?

No. The signatories certify the securities "were not acquired and are not held for the purpose of or with the effect of changing or influencing the control of the issuer."

What role does Oleg Nodelman have in the filing?

Mr. Nodelman is identified as the control person of EcoR1 and reports the same 66,844,275 Ordinary Shares shared voting and dispositive power, representing 8.4% of the class.