EXHIBIT A The undersigned, Ziff Asset Management, L.P., a Delaware limited partnership, PBK Holdings, Inc., a Delaware corporation, and Philip B. Korsant, hereby agree and acknowledge that the information required by this Schedule 13G, to which this...

                                   EXHIBIT A


      The  undersigned,   Ziff  Asset  Management,  L.P.,  a  Delaware  limited
partnership,  PBK  Holdings,  Inc.,  a  Delaware  corporation,  and  Xxxxxx  X.
Xxxxxxx,  hereby agree and acknowledge  that the  information  required by this
Schedule  13G, to which this  Agreement is attached as an exhibit,  is filed on
behalf  of each of  them.  The  undersigned  further  agree  that  any  further
amendments  or  supplements  thereto  shall  also be filed on behalf of each of
them.

Dated:  May 28, 2004

                                    ZIFF ASSET MANAGEMENT, L.P.
                                    By:PBK Holdings, Inc., its general partner


                                    By:   /s/ Xxxxxxxxx X. Xxxxx
                                       ------------------------------
                                       Name:  Xxxxxxxxx X. Xxxxx
                                       Title:  Vice President


                                    PBK HOLDINGS, INC.


                                    By:   /s/ Xxxxxxxxx X. Xxxxx
                                       ------------------------------
                                       Name:  Xxxxxxxxx X. Xxxxx
                                       Title:  Vice President



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