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