EcoR1 Group (NASDAQ: ATRA) reports ~17% holdings in Atara Biotherapeutics

Rhea-AI Filing Summary

EcoR1 Capital and related filers reported passive ownership stakes in Atara Biotherapeutics. The filing shows EcoR1 Capital, LLC and Oleg Nodelman each report 1,552,000 shares (representing 17.4% of the class) and EcoR1 Capital Fund Qualified, L.P. reports 1,445,609 shares (16.2%), based on 8,912,272 shares outstanding as of May 7, 2026. The filing states excluded warrants to acquire 1,090,922 and 1,026,558 shares respectively that are subject to a 9.99% beneficial ownership limitation and currently cannot be exercised.

Key Figures

EcoR1 shares held: 1,552,000 shares EcoR1 percent: 17.4% Qualified Fund shares: 1,445,609 shares +4 more

7 metrics

EcoR1 shares held 1,552,000 shares Amount beneficially owned by EcoR1 Capital and Oleg Nodelman

EcoR1 percent 17.4% Percent of class for EcoR1/Oleg Nodelman based on shares outstanding as of May 7, 2026

Qualified Fund shares 1,445,609 shares Amount beneficially owned by EcoR1 Capital Fund Qualified, L.P.

Qualified Fund percent 16.2% Percent of class for Qualified Fund based on May 7, 2026 outstanding shares

Shares outstanding (anchor) 8,912,272 shares Shares outstanding used to calculate percentages as of May 7, 2026

Warrants excluded (EcoR1) 1,090,922 warrants Warrants to acquire shares excluded from reported beneficial ownership; subject to 9.99% limit

Warrants excluded (Qualified Fund) 1,026,558 warrants Warrants to acquire shares excluded from reported beneficial ownership; subject to 9.99% limit

Key Terms

Schedule 13G/A, beneficial ownership limitation (9.99%), shared dispositive power, beneficially owned

4 terms

Schedule 13G/A regulatory

"Amendment No. 3; Schedule 13G/A filing header"

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.

beneficial ownership limitation (9.99%) regulatory

"subject to a 9.99% beneficial ownership limitation and currently cannot be exercised"

shared dispositive power financial

"Shared Dispositive Power 1,552,000.00 reported for EcoR1 and Nodelman"

beneficially owned regulatory

"The shares reported herein consist of 1,552,000 shares of Common Stock beneficially owned"

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.

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

See more from StockTitan in Google Search and AI answers. Adds StockTitan as a preferred source · opens Google

Add on Google Not now

Accepted by SEC EDGAR 05/07/2026 - 07:26 PM Learn about SEC filing dates

FAQ

What stake did EcoR1 report in Atara Biotherapeutics (ATRA)?

EcoR1 Capital, LLC and Oleg Nodelman each reported 1,552,000 shares, equal to 17.4% of the outstanding common stock based on 8,912,272 shares as of May 7, 2026. This reflects reported shared voting and dispositive power.

Are there warrants included in the reported EcoR1 ownership for ATRA?

The filing excludes warrants: EcoR1-related holders note warrants to acquire 1,090,922 and 1,026,558 shares. Those warrants are subject to a 9.99% beneficial ownership limitation and are currently not exercisable.

How is voting and dispositive power reported for the EcoR1 group?

Each reporting person discloses 0 sole voting or dispositive power and reports shared voting and shared dispositive power over the reported share totals (e.g., 1,552,000 shared for EcoR1 and Nodelman).

Does the filing say EcoR1 seeks control of Atara?

The signing statement certifies the securities were not acquired to change or influence control and were not held in connection with transactions to change control; the reporting persons disclaim group membership for control purposes.

What date is used to calculate the ownership percentages in the filing?

Percentages are calculated using 8,912,272 shares outstanding as of May 7, 2026, which is the explicit time anchor provided in the ownership statements.

SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934

(Amendment No. 3)

Atara Biotherapeutics, Inc.

(Name of Issuer)

Common Stock

(Title of Class of Securities)

046513206

(CUSIP Number)

05/07/2026

(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): 046513206

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 1,552,000.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 1,552,000.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,552,000.00

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)

17.4 %

12

Type of Reporting Person (See Instructions)

IA, OO

Comment for Type of Reporting Person: The shares reported herein consist of 1,552,000 shares of Common Stock beneficially owned by the reporting persons, and exclude warrants to acquire 1,090,922 shares of Common Stock, which are subject to a 9.99% beneficial ownership limitation and currently cannot be exercised. The percentage is calculated based on 8,912,272 shares of Common Stock outstanding on May 7, 2026.

SCHEDULE 13G

CUSIP Number(s): 046513206

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 1,552,000.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 1,552,000.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,552,000.00

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)

17.4 %

12

Type of Reporting Person (See Instructions)

HC, IN

Comment for Type of Reporting Person: The shares reported herein consist of 1,552,000 shares of Common Stock beneficially owned by the reporting persons, and exclude warrants to acquire 1,090,922 shares of Common Stock, which are subject to a 9.99% beneficial ownership limitation and currently cannot be exercised. The percentage is calculated based on 8,912,272 shares of Common Stock outstanding on May 7, 2026.

SCHEDULE 13G

CUSIP Number(s): 046513206

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 1,445,609.00
7	Sole Dispositive Power 0.00
8	Shared Dispositive Power 1,445,609.00

9

Aggregate Amount Beneficially Owned by Each Reporting Person

1,445,609.00

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)

16.2 %

12

Type of Reporting Person (See Instructions)

PN

Comment for Type of Reporting Person: The shares reported herein consist of 1,445,609 shares of Common Stock beneficially owned by the reporting persons, and exclude warrants to acquire 1,026,558 shares of Common Stock, which are subject to a 9.99% beneficial ownership limitation and currently cannot be exercised. The percentage is calculated based on 8,912,272 shares of Common Stock outstanding on May 7, 2026.

SCHEDULE 13G

Item 1.
(a)	Name of issuer: Atara Biotherapeutics, Inc.
(b)	Address of issuer's principal executive offices: 1280 Rancho Conejo Boulevard, Thousand Oaks, CA 91320
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: Common Stock
(e)	CUSIP No.: 046513206
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: 1,552,000 Oleg Nodelman: 1,552,000 Qualified Fund: 1,445,609
(b)	Percent of class: EcoR1: 17.4% Oleg Nodelman: 17.4% Qualified Fund: 16.2%
(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: 1,552,000 Oleg Nodelman: 1,552,000 Qualified Fund: 1,445,609
	(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: 1,552,000 Oleg Nodelman: 1,552,000 Qualified Fund: 1,445,609
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:	05/07/2026

Oleg Nodelman

Signature:	/s/ Oleg Nodelman
Name/Title:	Reporting person
Date:	05/07/2026

EcoR1 Capital Fund Qualified, L.P.

Signature:	/s/ Oleg Nodelman
Name/Title:	Manager of the General Partner, EcoR1 Capital, LLC
Date:	05/07/2026

Exhibit Information

Exhibit 99 - Agreement Regarding Joint Filing of Statement on Schedule 13D or 13G