EcoR1 Capital, LLC 4

What Happened
EcoR1 Capital, LLC (reported as a 10% owner) reported an open-market purchase of 507,407 shares of Atara Biotherapeutics (ATRA) on 2026-05-07 at a weighted average price of $9.35, for a total reported value of $4,744,255. The filing notes that EcoR1 is the adviser and general partner to funds that hold the shares and that the purchases were made for those funds rather than by an individual executive.

Key Details

• Transaction date: 2026-05-07 (reported on Form 4 filed 2026-05-11)
• Transaction type/code: Purchase (open market) — Code P
• Shares purchased: 507,407
• Weighted average price: $9.35; price range across trades $8.85–$9.39 (reporting person will provide per-price breakdown on request)
• Total value: $4,744,255
• Purchaser breakdown: Qualified Fund (EcoR1 Capital Fund Qualified, L.P.) bought 474,780 of the shares (F2)
• Holdings after transaction: Qualified Fund held 1,445,609 shares (F4)
• Reporting persons: EcoR1 Capital, LLC; Oleg Nodelman; and EcoR1 Capital Fund Qualified, L.P. (F1). They filed jointly but disclaim membership in a group and disclaim beneficial ownership except to the extent of pecuniary interest.
• Filing timeliness: Form 4 filed on 2026-05-11 — appears timely (filed within the required period following the 2026-05-07 transaction).

Context
This is an institutional purchase by a reported 10% owner/adviser-managed fund, not an individual executive. Purchases by funds can reflect portfolio activity rather than a personal view; the filing discloses the fund-level ownership and provides a price-range breakdown on request for further detail.