Exhibit A
                                                                       ---------

     The undersigned acknowledge and agree that the foregoing Statement on
Schedule 13G is filed on behalf of each of the undersigned and that all
subsequent amendments to such statement on Schedule 13G shall be filed on behalf
of each of the undersigned without the necessity of filing additional joint
filing agreements. The undersigned acknowledge that each shall be responsible
for the timely filing of such amendments, and for the completeness and accuracy
of the information concerning it contained therein, but shall not be responsible
for the completeness and accuracy of the information concerning the others,
except to the extent that it knows or has reason to believe that such
information is inaccurate. This Agreement may be executed in any number of
counterparts and all of such counterparts taken together shall constitute one
and the same instrument.

Dated:   May 23, 2006
                                  WARBURG, PINCUS VENTURES, L.P.

                                  By:  Warburg Pincus Partners LLC,
                                       its General Partner

                                  By:  Warburg Pincus & Co.
                                       its Managing Member


                                  By:   /s/ Scott A. Arenare
                                        --------------------------------------
                                        Name:   Scott A. Arenare
                                        Title:  Partner


                                  WARBURG PINCUS & CO.


                                  By:   /s/ Scott A. Arenare
                                        --------------------------------------
                                        Name:   Scott A. Arenare
                                        Title:  Partner


                                  WARBURG PINCUS LLC


                                  By:    /s/ Scott A. Arenare
                                        --------------------------------------
                                        Name:   Scott A. Arenare
                                        Title:  Managing Director


                                  WARBURG PINCUS PARTNERS LLC

                                  By:  Warburg Pincus & Co.
                                       its Managing Member


                                  By:    /s/ Scott A. Arenare
                                        --------------------------------------
                                        Name:   Scott A. Arenare
                                        Title:  Partner