1 JOINT FILING AGREEMENT We, the undersigned, hereby express our agreement that the attached Schedule 13D (including all amendments thereto) is filed on behalf of each of the undersigned.

   1
                            JOINT FILING AGREEMENT


         We, the undersigned, hereby express our agreement that the attached
Schedule 13D (including all amendments thereto) is filed on behalf of each of
the undersigned.


Date: September  19, 1997
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                                                  BB BIOTECH AG
                                                  
                                                  
                                                  By: /s/ XXXX-XXXXX XXXX
                                                     ------------------------
                                                  Name: Xxxx-Xxxxx Xxxx
                                                  
                                                  By: /s/ XX. XXXXXX XXXXXXXX
                                                     ------------------------
                                                  Name: Xx. Xxxxxx Xxxxxxxx
                                                  
                                                  BIOTECH TARGET, S.A.
                                                  
                                                  
                                                  By: /s/ XX. XXXXXXX XXXXXX
                                                     ------------------------
                                                  Name: Xx. Xxxxxxx Xxxxxx
                                                  
                                                  
                                                  By: /s/ DR. XXXXXX XXXX
                                                     ------------------------
                                                  Name: Dr. Xxxxxx Xxxx