Exhibit 99.1 ------------

 
                                                                    Exhibit 99.1
                                                                    ------------


                                   AGREEMENT

          Pursuant to Rule 13d-1(k)(1) promulgated pursuant to the Securities
Exchange Act of 1934, as amended, the undersigned agree that the attached
Schedule 13G is being filed on behalf of each of the undersigned.

February 9, 1999               VENROCK ASSOCIATES


                                   By:  /s/ Xxx X. XxXxxxxxxx
                                        ---------------------------
                                            Xxx X. XxXxxxxxxx
                                            General Partner


February 9, 1999               VENROCK ASSOCIATES II, L.P.

 
                                   By:  /s/ Xxx X. XxXxxxxxxx
                                        ---------------------------
                                            Xxx X. XxXxxxxxxx
                                            General Partner


February 9, 1999               GENERAL PARTNERS
 
                                                           *
                                            -------------------------------
                                            Xxxxxxx X. Xxxxx

                                                           *
                                            -------------------------------
                                            Xxxxx X. Xxxxxxxx

                                                           *
                                            -------------------------------
                                            Xxxxxxx X. Xxxxxxxxx

                                                           *
                                            -------------------------------
                                            Xxx X. XxXxxxxxxx

                                                           *
                                            -------------------------------
                                            Xxx X. Xxxxxxxx

                                                           *
                                            -------------------------------
                                            Xxxxxxxxx X. Xxxxxxxxxxx

                                                           *
                                            -------------------------------
                                            Xxxxxxx Sun


                                 * By:  /s/ Xxx X. XxXxxxxxxx
                                        -------------------------------
                                            Xxx X. XxXxxxxxxx
                                            Attorney-in-Fact

                              Page 17 of 17 pages