EcoR1 Capital, LLC bought ATRA

Footnotes

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.; 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.