Common Stock, par value $0.0001 per share

Atara Biotherapeutics, Inc.

1280 Rancho Conejo Blvd, Thousand Oaks, CA, 91320

This Amendment No. 2 to Schedule 13D (this "Amendment") amends and supplements the Schedule 13D filed with the SEC on May 23, 2025, which was previously amended and supplemented by that Amendment No. 1 to Schedule 13D filed with the SEC on November 14, 2025 (collectively, the "Prior Schedule 13D"). This Amendment reports an increase in the beneficial ownership of Redmile Group, LLC ("Redmile"), Jeremy C. Green, Redmile Biopharma Investments II, L.P. ("RBI II"), and Redmile Strategic Long Only Trading Sub, Ltd. ("Redmile Long Only", and, collectively with Redmile, Mr. Green and RBI II, the "Reporting Persons") due solely to the increase in the aggregate number of the Issuer's outstanding Common Stock, which increased the number of shares of Common Stock issuable to the Reporting Persons upon exercise of certain pre-funded warrants to purchase Common Stock (the "Warrants") under the Beneficial Ownership Limitation described in Item 5(a) below. The Reporting Persons have not engaged in any transactions in the Issuer's securities since the Prior Schedule 13D. Capitalized terms used but not defined in this Amendment shall have the meanings set forth in the Prior Schedule 13D. Except as specifically amended by this Amendment, the Prior Schedule 13D is unchanged.

Item 2(b) of the Prior Schedule 13D is hereby amended and restated in its entirety as follows: Redmile: 900 Larkspur Landing Circle, Suite 270 Larkspur, California 94939 Jeremy C. Green: c/o Redmile Group, LLC (NY Office) 45 W. 27th Street, Floor 11 New York, NY 10001 RBI II: c/o Redmile Group, LLC 900 Larkspur Landing Circle, Suite 270 Larkspur, California 94939 Redmile Long Only: c/o Redmile Group, LLC 900 Larkspur Landing Circle, Suite 270 Larkspur, California 94939

Item 5(a) of the Prior Schedule 13D is hereby amended and restated in its entirety as follows: 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 142,631 shares of Common Stock directly held by RBI II and the 66,855 shares of Common Stock directly held by Redmile Long Only, and (ii) 509,293 shares of Common Stock issuable upon the exercise of the Warrants held by the Redmile Funds. The shares of Common Stock issuable upon the exercise of the Warrants represent the maximum number of shares of Common Stock that could be issued under the 9.99% beneficial ownership limitation (the "Beneficial Ownership Limitation"). Subject in each case to the Beneficial Ownership Limitation, the Redmile Funds directly hold 3,412,843 Warrants, including the 2,054,155 Warrants directly held by RBI II and the 827,581 Warrants directly held by Redmile Long Only. All of the reported securities are directly owned by certain investment vehicles for which Redmile is the investment manager (the "Redmile Funds"), including RBI II and Redmile Long Only. Redmile may be deemed to beneficially own these securities in its capacity as the investment manager with discretion to vote and dispose of the securities held by the Redmile Funds. Mr. Green also may be deemed to beneficially own these securities as the principal of Redmile. Redmile and Mr. Green each disclaim beneficial ownership of these securities, except to the extent of its or his pecuniary interest in such securities, if any. Percentage for each Reporting Person is based on the sum of: (i) 9,010,172 shares of Common Stock outstanding as of May 8, 2026, as reported by the Issuer in its Form 10-Q for the quarterly period ended March 31, 2026 filed with the Securities and Exchange Commission (the "SEC") on May 12, 2026, plus (ii) 509,293 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.

Item 5(b) of the Prior Schedule 13D is hereby amended and restated in its entirety as follows: Redmile: (1) Sole Voting Power: 0 (2) Shared Voting Power: 950,994 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 950,994 Jeremy C. Green: (1) Sole Voting Power: 0 (2) Shared Voting Power: 950,994 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 950,994 RBI II: (1) Sole Voting Power: 0 (2) Shared Voting Power: 651,924 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 651,924 Redmile Long Only: (1) Sole Voting Power: 0 (2) Shared Voting Power: 576,148 (3) Sole Dispositive Power: 0 (4) Shared Dispositive Power: 576,148