| Explanation of Responses: |
| (1) | The securities are directly held by Peter Svennilson, a former director of the Issuer who is a managing partner at The Column Group which invests through The Column Group II, LP ("TCG II LP"), Ponoi Capital, LP ("Ponoi LP"), Ponoi Capital II, LP ("Ponoi II LP"), The Column Group III, LP ("TCG III LP") and The Column Group III-A, LP ("TCG III-A LP," and, together with TCG II LP, Ponoi LP, Ponoi II LP and TCG III LP, the "Funds"), among other entities. Under the partnership agreements of the Funds and his agreement with The Column Group Management, LP ("TCGM LP"), Mr. Svennilson is deemed to hold the securities for the economic benefit of the Funds and TCGM LP. The Funds and their respective general partners, TCGM LP and the individual managing partners of such entities may be deemed indirect beneficial owners of the securities. Such reporting persons disclaim beneficial ownership of the securities except to the extent of their pecuniary interest therein. |
| (2) | The securities are directly held by Peter Svennilson and certain of the securities are indirectly held by other reporting persons. See footnote 1 and related text. Mr. Svennilson disclaims beneficial ownership of the securities except to the extent of his pecuniary interest therein. |
| (3) | Consists of 91 shares of Common Stock held directly by TCG III LP and 102 shares of Common Stock held directly by TCG III-A LP. |
| (4) | Consists of 723,362 shares of Common Stock held directly by TCG III LP and 816,902 shares of Common Stock held directly by TCG III-A LP. |
| (5) | The securities are directly held by TCG III LP, and indirectly held by The Column Group III GP, LP ("TCG III GP"), the general partner of TCG III LP. The managing partners of TCG III GP are David Goeddel, Peter Svennilson and Tim Kutzkey. The managing partners of TCG III GP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (6) | The securities are directly held by TCG III-A LP, and indirectly held by TCG III GP, the general partner of TCG III-A LP. The managing partners of TCG III GP are David Goeddel, Peter Svennilson and Tim Kutzkey. The managing partners of TCG III GP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (7) | Consists of 11,103,333 shares of Common Stock held directly by The Column Group, LP ("TCG LP"), 2,265,758 shares of Common Stock held directly by TCG II LP, 100,000 shares of Common Stock held directly by The Column Group GP, LP ("TCG GP"),100,000 shares of Common Stock held directly by TCGM LP, 1,298,908 shares of Common Stock held directly by Ponoi LP, and 1,298,908 shares of Common Stock held directly by Ponoi II LP. |
| (8) | The securities are directly held by TCG LP, and indirectly held by TCG GP, the general partner of TCG LP. The managing partners of TCG GP are David Goeddel and Peter Svennilson. The managing partners of TCG GP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (9) | The securities are directly held by TCG II LP, and indirectly held by The Column Group II GP, LP ("TCG II GP"), the general partner of TCG II LP. The managing partners of TCG II GP are David Goeddel and Peter Svennilson. The managing partners of TCG II GP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (10) | The securities are directly held by TCG GP. The managing partners of TCG GP are David Goeddel and Peter Svennilson. The managing partners of TCG GP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (11) | The securities are directly held by TCGM LP. The managing partners of TCGM LP are David Goeddel and Peter Svennilson. The managing partners of TCGM LP may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (12) | The securities are directly held by Ponoi LP, and indirectly held by Ponoi Management, LLC ("Ponoi LLC"), the general partner of Ponoi LP. The managing partners of Ponoi LLC are David Goeddel, Peter Svennilson and Tim Kutzkey. The managing partners of Ponoi LLC may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (13) | The securities are directly held by Ponoi II LP, and indirectly held by Ponoi II Management, LLC ("Ponoi II LLC"), the general partner of Ponoi II LP. The managing partners of Ponoi II LLC are David Goeddel, Peter Svennilson and Tim Kutzkey. The managing partners of Ponoi II LLC may be deemed to have voting and investment power with respect to such shares. Each individual managing partner disclaims beneficial ownership of these shares, except to the extent of his pecuniary interest in such shares. |
| (14) | The securities are owned solely by Tim Kutzkey. |
| (15) | The securities are directly held by the David V. Goeddel and Alena Z. Goeddel 2004 Trust for which David V. Goeddel and Alena Z. Goeddel serve as co-trustee. |
| (16) | Issued pursuant to the Issuer's Amended and Restated 2018 Equity Incentive Plan. |
| (17) | The securities are directly held by David Goeddel, a director of the Issuer who is a managing partner at The Column Group which invests through the Funds, among other entities. Under the partnership agreements of the Funds and his agreement with TCGM LP, Mr. Goeddel is deemed to hold the securities for the economic benefit of the Funds and TCGM LP. The Funds and their respective general partners, TCGM LP and the individual managing partners of such entities may be deemed indirect beneficial owners of the securities. Such reporting persons disclaim beneficial ownership of the securities except to the extent of their pecuniary interest therein. |
| (18) | The option expires 90 days from May 20, 2020, the date Mr. Svennilson's service as a director of the Issuer ceased. |
| (19) | The stock option shall vest in four (4) approximately equal quarterly installments, such that the option shall be fully vested on the earlier of (i) the one-year anniversary of the date of grant and (ii) the day prior to the next Issuer's annual stockholder meeting, subject to Mr. Goeddel's Continuous Service (as defined in the Amended and Restated 2018 Equity Incentive Plan) on each applicable vesting date. |