Common Stock, par value $0.0001 per share

Atara Biotherapeutics, Inc.

1280 Rancho Conejo Blvd, Thousand Oaks, CA, 91320

This Schedule 13D is being filed jointly by Redmile Group, LLC ("Redmile"), Jeremy C. Green, RedCo I, L.P. ("RedCo I"), and Redmile Biopharma Investments II, L.P. ("RBI II" and collectively with Redmile, Mr. Green and RedCo I, the "Reporting Persons"), pursuant to the provisions of Rule 13d-1(k)(1) under the Securities Exchange Act of 1934, as amended (the "Exchange Act"), as separate persons and not as members of a group. See Exhibit 99.1 to this Schedule 13D for their Joint Filing Agreement.

Redmile: One Letterman Drive Building D, Suite D3-300 The Presidio of San Francisco San Francisco, California 94129 Jeremy C. Green: c/o Redmile Group, LLC (NY Office) 45 W. 27th Street, Floor 11 New York, NY 10001 RedCo I: c/o Redmile Group, LLC One Letterman Drive Building D, Suite D3-300 The Presidio of San Francisco San Francisco, California 94129 RBI II: c/o Redmile Group, LLC One Letterman Drive Building D, Suite D3-300 The Presidio of San Francisco San Francisco, California 94129

Redmile: investment manager/adviser to certain private investment vehicles and separately managed accounts. Jeremy C. Green: managing member of Redmile. RedCo I: private investment fund RBI II: private investment fund

During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and as a result of such proceeding was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Redmile: Delaware Jeremy C. Green: United Kingdom RedCo I: Delaware RBI II: Delaware

The aggregate amount of shares of Common Stock that may be deemed beneficially owned by the Reporting Persons is comprised of (i) 441,701 shares of Common Stock held by the Redmile Funds, including the 191,226 shares of Common Stock directly held by RedCo I and the 142,631 shares of Common Stock directly held by RBI II, and (ii) subject to the Beneficial Ownership Blocker, 3,412,843 shares of Common Stock issuable upon exercise of certain pre-funded warrants to purchase Common Stock (the "Warrants"), including the 156,216 shares of Common Stock issuable upon exercise of the Warrants directly held by RedCo I and the 2,054,155 shares of Common Stock issuable upon exercise of the Warrants directly held by RBI II. Redmile is the investment manager to the Redmile Funds and, in such capacity, exercises voting and investment power over all of the shares held by the Redmile Funds and may be deemed to be the beneficial owner of these shares. Jeremy C. Green serves as the managing member of Redmile and also may be deemed to be the beneficial owner of these shares. Redmile and Mr. Green each disclaims beneficial ownership of these shares, except to the extent of its or his pecuniary interest in such shares, if any. Pursuant to the terms of the Warrants, the Issuer may not effect any exercise of the Warrants, and a holder of a Warrants does not have the right to exercise the Warrants held by such holder, to the extent that after giving effect to such issuance after exercise, the holder (together with the holder's affiliates, and any other persons acting as a group together with the holder or any of the holder's affiliates), would beneficially own in excess of 9.99% (the "Beneficial Ownership Limitation") of the number of shares of Common Stock outstanding immediately after giving effect to the issuance of shares of Common Stock issued upon exercise of the Warrant (the "Beneficial Ownership Blocker"). The Beneficial Ownership Limitation may be changed at a holder's election upon 61 days' notice to the Issuer. The 705,207 shares of Common Stock reported as beneficially owned by Redmile and Mr. Green in this Schedule 13D represent 9.99% of the outstanding shares of Common Stock. The 347,442 shares of Common Stock reported as beneficially owned by RedCo I in this Schedule 13D represent 5.0% of the outstanding shares of Common Stock. The 406,137 shares of Common Stock reported as beneficially owned by RBI II in this Schedule 13D represent 5.8% of the outstanding shares of Common Stock. For purposes of this Schedule 13D, the percent of class for each Reporting Person was calculated based on sum of: (i) 5,961,391 shares of Common Stock outstanding as of May 7, 2025, as reported by the Issuer in its Form 10-Q for the quarterly period ended March 31, 2025 filed with the SEC on May 15, 2025; plus (ii) 834,237 shares of Common Stock issued in the Registered Offering as reported by the Issuer in its Form 8-K dated May 14, 2025 filed with the SEC on May 16, 2025, plus (iii) 263,506 shares of Common Stock issuable upon exercise of the Warrants directly held by the Redmile Funds, which due to the Beneficial Ownership Limitation is the maximum number of shares that could be issued upon exercise of those Warrants.

Redmile: (1) Sole Voting Power: 0 (2) Shared Voting Power: 705,207 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 705,207 Jeremy C. Green: (1) Sole Voting Power: 0 (2) Shared Voting Power: 705,207 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 705,207 RedCo I: (1) Sole Voting Power: 0 (2) Shared Voting Power: 347,442 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 347,442 RBI II: (1) Sole Voting Power: 0 (2) Shared Voting Power: 406,137 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 406,137

See the response to Item 3. Except as disclosed in Item 3, no reportable transactions were effected by any Reporting Persons during the past sixty days.

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