EXHIBIT 4 Joint Filing Agreement We, the signatories of the Statement on Schedule 13D to which this Joint filing Agreement is attached, hereby agree that such Statement is filed, and any amendments thereto filed by either or both of us will be filed,...

                                    EXHIBIT 4

                             Joint Filing Agreement


         We, the  signatories  of the  Statement  on Schedule  13D to which this
Joint filing  Agreement is attached,  hereby agree that such Statement is filed,
and any  amendments  thereto  filed by either  or both of us will be  filed,  on
behalf of each of us.

         Dated: June 21, 1999.

                          VULCAN VENTURES INCORPORATED


                          By: /s/ Xxxxxxx X. Xxxxx
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                              Xxxxxxx X. Xxxxx, Vice President


                              /s/ Xxxx X. Xxxxx
                              ----------------------------------------------
                              Xxxx X. Xxxxx