EcoR1 Capital (NASDAQ: ATRA) adds 507K Atara shares in open-market buy

Rhea-AI Filing Summary

EcoR1 Capital, LLC, a ten percent owner of Atara Biotherapeutics, reported an indirect open-market purchase of 507,407 shares of Common Stock at a weighted average price of $9.35 per share. The shares were bought by funds it advises, which now hold 1,552,000 shares in total.

Insider Trade Summary

Net Buyer: 507,407 shares ($4,744,255)

Net Buy 1 txn

Insider EcoR1 Capital, LLC

Role null

Bought 507,407 shs ($4.74M)

Type	Security	Shares	Price	Value
Purchase	Common Stock	507,407	$9.35	$4.74M

Holdings After Transaction: Common Stock — 1,552,000 shares (Indirect, See Note)

Footnotes (1)

1. The reporting persons are EcoR1 Capital, LLC ("EcoR1"), Oleg Nodelman and EcoR1 Capital Fund Qualified, L.P. ("Qualified Fund"). EcoR1 is the general partner and investment adviser of private funds, including Qualified Fund (the "Funds"). Mr. Nodelman is the manager and controller owner of EcoR1. EcoR1 is filing this Form 4 for itself, Mr. Nodelman and Qualified Fund. The reporting persons are filing this Form 4 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 the securities except to the extent of their respective pecuniary interests therein. Qualified Fund purchased 474,780 of the shares puchased in this transaction. The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $8.85 to $9.39 inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in this footnote. After this transaction, Qualified Fund held 1,445,609 shares of the Issuer's Common Stock.

Key Figures

Shares purchased: 507,407 shares Weighted average price: $9.35 per share Price range: $8.85–$9.39 per share +3 more

6 metrics

Shares purchased 507,407 shares Common Stock open-market purchase

Weighted average price $9.35 per share Purchase price for Common Stock

Price range $8.85–$9.39 per share Range of individual trade prices

Total shares after transaction 1,552,000 shares Indirect holdings following purchase

Qualified Fund portion 474,780 shares Part of total purchased by Qualified Fund

Qualified Fund holdings 1,445,609 shares Qualified Fund position after transaction

Key Terms

open-market purchase, indirect beneficial ownership, ten percent owner, weighted average price, +1 more

5 terms

open-market purchase financial

"transaction_action: open-market purchase"

An open-market purchase is when an investor or a company buys shares on a public stock exchange at the going market price, rather than through a private deal. It matters to investors because these purchases change how many shares are available, can push the stock price up or signal confidence from large buyers, and often affect per-share metrics like earnings—think of it like someone buying lots of apples off a grocery shelf, reducing supply and potentially raising the price.

indirect beneficial ownership financial

"EcoR1 may be deemed to indirectly beneficially own them as the investment adviser"

ten percent owner financial

"EcoR1 Capital, LLC is indicated as a ten percent owner"

weighted average price financial

"The price reported in Column 4 is a weighted average price."

Weighted average price is the average price of a security where each trade or component is counted according to its size, so bigger trades pull the average more than smaller ones. Think of it like calculating the average cost of a grocery haul where items you bought more of have greater influence on the final per-item cost. Investors use it to understand the true average price paid or received, judge execution quality, and compare trading performance against market movement.

pecuniary interests financial

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

Form 4 Insider Transactions: What Each Field Means →

SEC Form 4

FORM 4

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

STATEMENT OF CHANGES IN BENEFICIAL OWNERSHIP

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:			3235-0287
Estimated average burden
hours per response:			0.5

Check this box if no longer subject to Section 16. Form 4 or Form 5 obligations may continue. See Instruction 1(b).

Check this box to indicate that a transaction was made pursuant to a contract, instruction or written plan for the purchase or sale of equity securities of the issuer that is intended to satisfy the affirmative defense conditions of Rule 10b5-1(c). See Instruction 10.

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

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

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

2a. Foreign Trading Symbol

3. Date of Earliest Transaction (Month/Day/Year)
05/07/2026

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

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

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

Table I - Non-Derivative Securities Acquired, Disposed of, or Beneficially Owned
1. Title of Security (Instr. 3)	2. Transaction Date (Month/Day/Year)	2A. Deemed Execution Date, if any (Month/Day/Year)	3. Transaction Code (Instr. 8)		4. Securities Acquired (A) or Disposed Of (D) (Instr. 3, 4 and 5)			5. Amount of Securities Beneficially Owned Following Reported Transaction(s) (Instr. 3 and 4)	6. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	7. Nature of Indirect Beneficial Ownership (Instr. 4)
			Code	V	Amount	(A) or (D)	Price
Common Stock	05/07/2026		P		507,407(2)	A	$9.35(3)	1,552,000(4)	I	See Note(1)

Table II - Derivative Securities Acquired, Disposed of, or Beneficially Owned (e.g., puts, calls, warrants, options, convertible securities)
1. Title of Derivative Security (Instr. 3)	2. Conversion or Exercise Price of Derivative Security	3. Transaction Date (Month/Day/Year)	3A. Deemed Execution Date, if any (Month/Day/Year)	4. Transaction Code (Instr. 8)		5. Number of Derivative Securities Acquired (A) or Disposed of (D) (Instr. 3, 4 and 5)		6. Date Exercisable and Expiration Date (Month/Day/Year)		7. Title and Amount of Securities Underlying Derivative Security (Instr. 3 and 4)		8. Price of Derivative Security (Instr. 5)	9. Number of derivative Securities Beneficially Owned Following Reported Transaction(s) (Instr. 4)	10. Ownership Form: Direct (D) or Indirect (I) (Instr. 4)	11. Nature of Indirect Beneficial Ownership (Instr. 4)
				Code	V	(A)	(D)	Date Exercisable	Expiration Date	Title	Amount or Number of Shares

Explanation of Responses:

1. The reporting persons are EcoR1 Capital, LLC ("EcoR1"), Oleg Nodelman and EcoR1 Capital Fund Qualified, L.P. ("Qualified Fund"). EcoR1 is the general partner and investment adviser of private funds, including Qualified Fund (the "Funds"). Mr. Nodelman is the manager and controller owner of EcoR1. EcoR1 is filing this Form 4 for itself, Mr. Nodelman and Qualified Fund. The reporting persons are filing this Form 4 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 the securities except to the extent of their respective pecuniary interests therein.

2. Qualified Fund purchased 474,780 of the shares puchased in this transaction.

3. The price reported in Column 4 is a weighted average price. These shares were purchased in multiple transactions at prices ranging from $8.85 to $9.39 inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission upon request, full information regarding the number of shares purchased at each separate price within the ranges set forth in this footnote.

4. After this transaction, Qualified Fund held 1,445,609 shares of the Issuer's 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 4 (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 4: SEC 1474 (03-26)

FAQ

How many Atara Biotherapeutics (ATRA) shares did EcoR1 Capital buy?

EcoR1-linked funds bought 507,407 Atara Biotherapeutics shares in an open-market transaction. This increased their indirect ownership position, with total shares held reaching 1,552,000 after the purchase, according to the reported Common Stock transaction data.

What price did EcoR1 Capital pay for ATRA shares in this transaction?

The reported weighted average purchase price was $9.35 per Atara Biotherapeutics share. Footnotes note multiple trades between $8.85 and $9.39, with EcoR1 undertaking to provide detailed breakdowns of the individual trade prices upon request.

Which EcoR1 fund bought most of the Atara Biotherapeutics (ATRA) shares?

EcoR1 Capital Fund Qualified, L.P. purchased 474,780 of the 507,407 shares bought in this transaction. A footnote states that after the trade, Qualified Fund held 1,445,609 Atara Biotherapeutics Common Stock shares for the benefit of its investors.

How many Atara Biotherapeutics (ATRA) shares do EcoR1-related funds hold after the trade?

After this purchase, funds advised by EcoR1 Capital are reported as holding 1,552,000 Atara Biotherapeutics Common Stock shares indirectly. This figure reflects total shares following the open-market transaction disclosed for EcoR1’s associated investment vehicles.

Does EcoR1 Capital have direct ownership of the ATRA shares it reported?

The shares are held directly by investment funds, including Qualified Fund, for their investors. EcoR1 and Oleg Nodelman may be deemed to beneficially own them indirectly but expressly disclaim beneficial ownership except to the extent of their pecuniary interests.