EXHIBIT A AGREEMENT REGARDING JOINT FILING The undersigned, Biotechnology Value Fund, L.P., a Delaware limited partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF Inc., a Delaware corporation, hereby agree and acknowledge that...

                                                                 EXHIBIT 99(A)

                                     EXHIBIT A

                           AGREEMENT REGARDING JOINT FILING


     The undersigned, Biotechnology Value Fund, L.P., a Delaware limited
partnership, and BVF Partners L.P., a Delaware limited partnership, and BVF
Inc., a Delaware corporation, hereby agree and acknowledge that the information
required by Amendment 1 to Schedule 13G, to which this Agreement is attached as
an exhibit, is filed on behalf of each of them.  The undersigned further agree
that any amendments or supplements thereto shall also be filed on behalf of each
of them.

Dated:  June 8, 1999

               BIOTECHNOLOGY VALUE FUND, L.P.

               By:  BVF Partners L.P., its general partner

                    By:  BVF Inc., its general partner


                         By: /s/ Xxxx X. Xxxxxxx
                             -------------------
                             Xxxx X. Xxxxxxx
                             President

               BVF PARTNERS L.P.

               By:  BVF Inc., its general partner


                    By:  /s/ Xxxx X. Xxxxxxx
                         -------------------
                         Xxxx X. Xxxxxxx
                         President

               BVF INC.


               By: /s/ Xxxx X. Xxxxxxx
                   -------------------
                   Xxxx X. Xxxxxxx
                   President