EcoR1 discloses Atara stake in Form 3

EcoR1 Capital, LLC filed an initial Form 3 reporting its status as a more than ten percent owner of Atara Biotherapeutics, Inc. common stock.

Filing Impact

(Neutral)

Filing Sentiment

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Sentiment and the balance of points

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Form Type

Rhea-AI Filing Summary

EcoR1 Capital, LLC filed an initial Form 3 reporting its status as a more than ten percent owner of Atara Biotherapeutics, Inc. common stock. The filing shows indirect beneficial ownership of 1,044,593 shares, with EcoR1-related Qualified Fund holding 970,829 shares for the benefit of its investors.

Insider Trade Summary

1 transaction reported

Mixed 1 txn

Insider EcoR1 Capital, LLC

Role 10% Owner

Type	Security	Shares	Price	Value
holding	Common Stock	--	--	--

Holdings After Transaction: Common Stock — 1,044,593 shares (Indirect, See Note)

Footnotes (2)

F1. The reporting persons are EcoR1 Capital, LLC ("EcoR1"), EcoR1 Capital Fund Qualified L.P. ("Qualified Fund") and Oleg Nodelman. EcoR1 is the general partner and investment adviser of private funds, including Qualified Fund (the "Funds"). Mr. Nodelman is the manager and controlling owner of EcoR1. EcoR1 is filing this Form 3 for itself, Mr. Nodelman and Qualified Fund. The reporting persons are filing this Form 3 jointly, but not as a group, and each expressly disclaims membership in a group within the meaning of Rule 13d-5(b) under the Securities Exchange Act of 1934. The Funds hold these securities directly for the benefit of their investors. EcoR1 may be deemed to indirectly beneficially own them as the investment adviser to the Funds. Mr. Nodelman may be deemed to indirectly beneficially own them as the control person of EcoR1. The reporting persons disclaim beneficial ownership of such securities except to the extent of their respective pecuniary interests therein.
F2. Qualified Fund beneficially owns 970,829 shares of Common Stock.

Key Figures

Indirect beneficial ownership: 1,044,593 shares Qualified Fund holdings: 970,829 shares Ten percent owner status: More than 10% ownership

3 metrics

Indirect beneficial ownership 1,044,593 shares Total Atara common stock indirectly owned following Form 3

Qualified Fund holdings 970,829 shares Atara common stock beneficially owned by EcoR1 Capital Fund Qualified L.P.

Ten percent owner status More than 10% ownership EcoR1 Capital identified as over ten percent owner of Atara

Key Terms

ten percent owner, beneficial ownership, pecuniary interests, Rule 13d-5(b)

4 terms

ten percent owner regulatory

"EcoR1 Capital, LLC is identified as a more than ten percent owner"

beneficial ownership financial

"Qualified Fund beneficially owns 970,829 shares of Common Stock"

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.

pecuniary interests financial

"The reporting persons disclaim beneficial ownership except to the extent of their respective pecuniary interests therein"

Rule 13d-5(b) regulatory

"each expressly disclaims membership in a group within the meaning of Rule 13d-5(b) under the Securities Exchange Act of 1934"

FAQ

AI-generated questions and answers. How Rhea-AI works. Not financial advice.

What does EcoR1 Capital report in its Atara (ATRA) Form 3 filing?

EcoR1 Capital, LLC reports initial beneficial ownership in Atara Biotherapeutics common stock. The filing shows EcoR1-associated funds indirectly own 1,044,593 shares, including 970,829 shares held by EcoR1 Capital Fund Qualified L.P. for the benefit of its investors.

Is EcoR1 Capital a ten percent owner of Atara Biotherapeutics (ATRA)?

Yes. EcoR1 Capital, LLC is identified as a more than ten percent beneficial owner of Atara Biotherapeutics. The Form 3 reflects indirect ownership through investment funds it advises, rather than direct personal holdings of EcoR1 or its manager, Oleg Nodelman.

How many Atara (ATRA) shares does EcoR1 Capital’s Qualified Fund hold?

EcoR1 Capital Fund Qualified L.P. beneficially owns 970,829 shares of Atara Biotherapeutics common stock. These shares are held directly by the fund for its investors, while EcoR1 and Oleg Nodelman may be deemed indirect beneficial owners through their advisory and control roles.

Does the EcoR1 Capital Form 3 for Atara (ATRA) show any recent buy or sell transactions?

No specific buy or sell transactions are reported. The Form 3 functions as an initial ownership statement, listing EcoR1-related funds’ existing indirect holdings in Atara Biotherapeutics common stock rather than reporting new purchases or sales.

How does EcoR1 Capital describe its beneficial ownership of Atara (ATRA) shares?

EcoR1 states that the funds hold the securities directly for their investors, and EcoR1 and Oleg Nodelman may be deemed indirect beneficial owners. They expressly disclaim beneficial ownership beyond their respective pecuniary interests in these Atara Biotherapeutics shares.

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

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Accepted by SEC EDGAR 05/11/2026 - 05:33 PM EDT

SEC Form 3

FORM 3

UNITED STATES SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

INITIAL STATEMENT OF BENEFICIAL OWNERSHIP OF SECURITIES

Filed pursuant to Section 16(a) of the Securities Exchange Act of 1934
or Section 30(h) of the Investment Company Act of 1940

OMB APPROVAL

OMB Number:			3235-0104
Estimated average burden
hours per response:			0.5

1. Name and Address of Reporting Person^*

EcoR1 Capital, LLC

(Last)

(First)

(Middle)

357 TEHAMA STREET #3

(Street)

SAN FRANCISCO

CALIFORNIA

94103

(City)

(State)

(Zip)

UNITED STATES

(Country)

2. Date of Event Requiring Statement (Month/Day/Year)
05/07/2026

3. Issuer Name and Ticker or Trading Symbol
Atara Biotherapeutics, Inc. [ ATRA ]

3a. Foreign Trading Symbol

5. If Amendment, Date of Original Filed (Month/Day/Year)

4. Relationship of Reporting Person(s) to Issuer
(Check all applicable)

Director	X	10% Owner
Officer (give title below)		Other (specify below)

6. Individual or Joint/Group Filing (Check Applicable Line)

X	Form filed by One Reporting Person
	Form filed by More than One Reporting Person

Table I - Non-Derivative Securities Beneficially Owned
1. Title of Security (Instr. 4)	2. Amount of Securities Beneficially Owned (Instr. 4)	3. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	4. Nature of Indirect Beneficial Ownership (Instr. 5)
Common Stock	1,044,593⁽²⁾	I	See Note⁽¹⁾

Table II - Derivative Securities Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 4)	2. Date Exercisable and Expiration Date (Month/Day/Year)		3. Title and Amount of Securities Underlying Derivative Security (Instr. 4)		4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)
1. Title of Derivative Security (Instr. 4)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares	4. Conversion or Exercise Price of Derivative Security	5. Ownership Form: Direct (D) or Indirect (I) (Instr. 5)	6. Nature of Indirect Beneficial Ownership (Instr. 5)

Explanation of Responses:

1. The reporting persons are EcoR1 Capital, LLC ("EcoR1"), EcoR1 Capital Fund Qualified L.P. ("Qualified Fund") and Oleg Nodelman. EcoR1 is the general partner and investment adviser of private funds, including Qualified Fund (the "Funds"). Mr. Nodelman is the manager and controlling owner of EcoR1. EcoR1 is filing this Form 3 for itself, Mr. Nodelman and Qualified Fund. The reporting persons are filing this Form 3 jointly, but not as a group, and each expressly disclaims membership in a group within the meaning of Rule 13d-5(b) under the Securities Exchange Act of 1934. The Funds hold these securities directly for the benefit of their investors. EcoR1 may be deemed to indirectly beneficially own them as the investment adviser to the Funds. Mr. Nodelman may be deemed to indirectly beneficially own them as the control person of EcoR1. The reporting persons disclaim beneficial ownership of such securities except to the extent of their respective pecuniary interests therein.

2. Qualified Fund beneficially owns 970,829 shares of Common Stock.

	/s/ Oleg Nodelman, Manager of EcoR1 Capital, LLC	05/11/2026
	** Signature of Reporting Person	Date
Reminder: Report on a separate line for each class of securities beneficially owned directly or indirectly.
* If the form is filed by more than one reporting person, see Instruction 5 (b)(v).
** Intentional misstatements or omissions of facts constitute Federal Criminal Violations See 18 U.S.C. 1001 and 15 U.S.C. 78ff(a).
Note: File three copies of this Form, one of which must be manually signed. If space is insufficient, see Instruction 6 for procedure.
Persons who respond to the collection of information contained in this form are not required to respond unless the form displays a currently valid OMB Number.
* Form 3: SEC 1473 (03-26)

Source: View Original Filing on SEC EDGAR