Page 20 of 20 pages

                                                             Page 20 of 20 pages

                                                                    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 14, 2001                           VENROCK ASSOCIATES


                                            By: /s/ Xxxxxxx X. Xxxxx
                                                -----------------------------
                                                Xxxxxxx X. Xxxxx
                                                General Partner

February 14, 2001                           VENROCK ASSOCIATES II, L.P.


                                            By: /s/ Xxxxxxx X. Xxxxx
                                                -----------------------------
                                                Xxxxxxx X. Xxxxx
                                                General Partner

February 14, 2001                           GENERAL PARTNERS


                                            /s/Xxxxxxx X. Xxxxxx
                                            ----------------------------------
                                            Xxxxxxx X. Xxxxxx

                                            /s/Xxxxxx X. Xxxxx
                                            ----------------------------------
                                            Xxxxxx X. Xxxxx

                                            /s/Xxxx Xxxxxxxx
                                            ----------------------------------
                                            Xxxx Xxxxxxxx

                                            /s/Xxxxxxx X. Xxxxx
                                            ----------------------------------
                                            Xxxxxxx X. Xxxxx

                                            /s/Xxxxxx X. Xxxxxxxxx
                                            ----------------------------------
                                            Xxxxxx X. Xxxxxxxxx

                                            /s/Xxxxx X. Xxxxxxxx
                                            ----------------------------------
                                            Xxxxx X. Xxxxxxxx

                                            /s/Xxxxxxx X. Xxxxxxxxx
                                            ----------------------------------
                                            Xxxxxxx X. Xxxxxxxxx

                                            /s/Xxx X. XxXxxxxxxx
                                            ----------------------------------
                                            Xxx X. XxXxxxxxxx

                                            /s/Xxx X. Xxxxxxxx
                                            ----------------------------------
                                            Xxx X. Xxxxxxxx

                                            /s/Xxxxxxxxx X. Xxxxxxxxxxx
                                            ----------------------------------
                                            Xxxxxxxxx X. Xxxxxxxxxxx

                                            /s/Xxxxxxx Sun
                                            ----------------------------------
                                            Xxxxxxx Sun

                                            /s/Xxxxxxx X. Xxxxxxx
                                            ----------------------------------
                                            Xxxxxxx X. Xxxxxxx