JOINT FILING AGREEMENT

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                                    EXHIBIT 4

                             JOINT FILING AGREEMENT

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

                                     BB BIOTECH AG

Date:   May 26, 1998                 By:    /s/ Xxxx-Xxxxx Xxxx
                                            --------------------
                                     Name:  Xxxx-Xxxxx Xxxx
                                            Signatory Authority

Date:   May 26, 1998                 By:    /s/ Xx. Xxxxxx Xxxxxxxx
                                            ------------------------
                                     Name:  Xx. Xxxxxx Xxxxxxxx
                                            --------------------
                                            Vice Chairman and Director

                                     BIOTECH TARGET, S.A.

Date:   May 26, 1998                 By:    /s/ Xx. Xxxxxxx Xxxxxx
                                            -----------------------
                                     Name:  Xx. Xxxxxxx Xxxxxx
                                            --------------------
                                            Signatory Authority

Date:   May 26, 1998                 By:    /s/ Dr. Xxxxxx Xxxx
                                            --------------------
                                     Name:  Dr. Xxxxxx Xxxx
                                            --------------------
                                            Signatory Authority

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