Power of Attorney

       The undersigned, a managing partner of Ponoi Management, LLC, hereby
constitutes and appoints each of Gary A. Jungels, Jennifer J. Carlson, Jennifer
Brake and Cathy Franczyk, for so long as they are affiliated with a law firm
representing The Column Group, LLC, the undersigned's true and lawful attorney-
in-fact, each with the power to act alone for the undersigned and in the
undersigned's name, place and stead, to:

       1. Prepare, complete and execute an SEC Form ID required by Sections
          13(a) and 23(a) of the Securities Exchange Act of 1934, as amended
          (the "Exchange Act"), and the rules and regulations promulgated
          thereunder, and any and all amendments thereto, and any other
          documents necessary or appropriate in order to obtain EDGAR access
          codes, and to file or cause to be filed the same with the Securities
          and Exchange Commission as may be required or advisable;

      2. Prepare, complete and execute Forms 3, 4 and 5 under the Exchange Act
         and other forms and all amendments thereto on the undersigned's behalf
         as such attorney-in-fact shall in his or her discretion determine to be
         required or advisable pursuant to Section 16 or Section 13 of the
         Exchange Act and the rules and regulations promulgated thereunder, or
         any successor laws and regulations, as a consequence of the
         undersigned's beneficial ownership of securities or changes in such
         beneficial ownership; and

      3. Do all acts necessary in order to file such forms and amendments with
         the Securities and Exchange Commission, any securities exchange or
         national association and such other persons or agencies as the
         attorney-in-fact shall deem appropriate.

      The undersigned hereby ratifies and confirms all that said
attorneys-in-fact and agents shall do or cause to be done by virtue hereof. The
undersigned acknowledges that the foregoing attorneys-in-fact, in serving in
such capacity at the request of the undersigned, are not assuming any of the
undersigned's responsibilities to comply with or liability under Section 16 or
Section 13 of the Exchange Act or any rules and regulations promulgated
thereunder or any successor laws and regulations.

      This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4, or 5 or such other forms
and amendments thereto pursuant to Section 16 or Section 13 of the Exchange Act
with respect to the undersigned's beneficial ownership of securities or changes
in such beneficial ownership, unless earlier revoked by the undersigned in a
signed writing delivered to the foregoing attorneys-in-fact.

       The undersigned has executed this Power of Attorney as of
October 23, 2017.



                                          /s/ Peter Svennilson
                                          ------------------------------
                                          Managing Partner