1. Institut de Pharmacologie, Centre National de la Recherche Scientifique UPR 411, 06560 Valbonne, France; Institut National de la Sante et de la Recherche Medicale U.406, Faculté de Médecine de la Timone, 13385 Marseille Cedex, France; and Medical Research Council Laboratory for Molecular Cell Biology, Cancer Research Campaign Oncogene and Signal Transduction Group and Department of Biochemistry,