EXHIBIT A The undersigned, Ziff Asset Management, L.P., a Delaware limited partnership, PBK Holdings, Inc., a Delaware corporation, Philip B. Korsant, and ZBI Equities, L.L.C., a Delaware limited liability company, hereby agree and acknowledge that...

                                 EXHIBIT A


      The  undersigned,  Ziff  Asset  Management,  L.P.,  a  Delaware  limited
partnership,  PBK Holdings,  Inc., a Delaware corporation,  Xxxxxx X. Xxxxxxx,
and ZBI Equities,  L.L.C., a Delaware limited liability company,  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:  February 6, 2006

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


                                    By:/s/ XXXXX XXXX
                                       --------------------------------------
                                       Name:  Xxxxx Xxxx
                                       Title: Vice President


                                    PBK HOLDINGS, INC.


                                    By:/s/ XXXXX XXXX
                                       --------------------------------------
                                       Name:  Xxxxx Xxxx
                                       Title: Vice President



                                       /s/ XXXXXX X. XXXXXXX
                                       --------------------------------------
                                            Xxxxxx X. Xxxxxxx


                                    ZBI EQUITIES, L.L.C.
                                    By:PBK Holdings, Inc., its sole member


                                    By:/s/ XXXXX XXXX
                                       --------------------------------------
                                       Name:  Xxxxx Xxxx
                                       Title: Vice President