POWER OF ATTORNEY

     Know all by these present, that Millennium Pharmaceuticals, Inc. (the
"Reporting Person") hereby constitutes and appoints each of Paul Sundberg, Chad
Diehl and Brett Budzinski, signing singly, as the Reporting Person's true and
lawful attorney-in-fact to:

     (l)  prepare, execute in the Reporting Person's name and on the Reporting
          Person's behalf, and submit to the U.S. Securities and Exchange
          Commission (the "SEC") a Form ID, including amendments thereto, and
          any other documents necessary or appropriate to obtain codes and
          passwords enabling the Reporting Person to make electronic filings
          with the SEC of Reports required by Sections 13 and 16(a) of the
          Securities Exchange Act of 1934 or any rule or regulation of the SEC;

     (2)  prepare and file on behalf of the Reporting Person any and all
          reports, notices, communications and other documents (including, but
          not limited to, reports on Schedule 13D, Schedule 13G, Form 3, Form 4
          and Form 5) that the Reporting Person may be required to file with the
          SEC pursuant to the Securities Act of 1933, as amended (together with
          the implementing regulations thereto, the "Act") and the Securities
          Exchange Act of 1934, as amended (together with the implementing
          regulations thereto, the "Exchange Act") (collectively, the "Reports")
          with respect to the Reporting Person's ownership of, or transactions
          in, the securities of any entity whose securities are beneficially
          owned (directly or indirectly) by the Reporting Person ("Portfolio
          Companies");

     (3)  do and perform any and all acts for and on behalf of the Reporting
          Person which may be necessary or desirable to complete and execute any
          such Reports, complete and execute any amendment or amendments
          thereto, and timely file such form with the SEC and any stock exchange
          or similar authority;

     (4)  complete for and on behalf of the Reporting Person, execute in the
          Reporting Person's name and on the Reporting Person's behalf, and
          submit to the requestor thereof, any questionnaires, documents or
          other materials that are requested in connection with any (i) equity
          or debt offering by a Portfolio Company and (ii) exercise by the
          Reporting Person of voting or proxy rights in connection with the
          Reporting Person's equity ownership of any Portfolio Company; and

     (5)  take any other action of any type whatsoever in connection with the
          foregoing which, in the opinion of such attorney-in-fact, may be of
          benefit to, in the best interest of, or legally required by, the
          Reporting Person, it being understood that the documents executed by
          such attorney-in-fact on behalf of the Reporting Person pursuant to
          this Power of Attorney shall be in such form and shall contain such
          terms and conditions as such attorney-in-fact may approve in such
          attorney-in-fact's discretion.

     The Reporting Person hereby grants to each such attorney-in-fact full power
and authority to do and perform any and every act and thing whatsoever
requisite, necessary, or proper to be done in the exercise of any of the rights
and powers herein granted, as fully to all intents and purposes as the Reporting
Person might or could do if the legal representatives of the Reporting Person
were personally present, hereby ratifying and confirming all that such
attorney-in-fact shall lawfully do or cause to be done by virtue of this power
of attorney and the rights and powers herein granted. The Reporting Person
acknowledges that the foregoing attorneys-in-fact, in serving in such capacity
at the request of the Reporting Person, are not assuming nor relieving any of
the Reporting Person's responsibilities to comply with Section 13 or Section 16
of the Exchange Act. The Reporting Person acknowledges that none of the
foregoing attorneys-in-fact assume (i) any liability for the Reporting Person's
responsibility to comply with the requirements of the Act or the Exchange Act,
(ii) any liability of the Reporting Person for any failure to comply with such
requirements or (iii) any obligation or liability of the Reporting Person for
profit disgorgement under Section 16(b) of the Exchange Act.

     This Power of Attorney shall remain in full force and effect until the
Reporting Person is no longer required to file any Reports with respect to the
Reporting Person's ownership of, or transactions in, the securities of Portfolio
Companies, unless earlier revoked by the Reporting Person at any time at its
sole discretion. This Power of Attorney shall expire as to any individual
attorney-in-fact if such attorney-in-fact ceases to serve as an employee of
Takeda Pharmaceutical Company Limited or one of its subsidiaries.

                            [Signature page follows]

     IN WITNESS WHEREOF, the Reporting Person has caused this Power of Attorney
to become effective as of the date set forth below.


                                        MILLENNIUM PHARMACEUTICALS, INC.

                                        By: /s/ Fabien Dubois
                                           -------------------------
                                        Name: Fabien Dubois
                                             -----------------------
                                        Title: Head of Finance
                                              ----------------------
                                        Date: 12/14/17
                                             -----------------------