1. Division of Hormone Research, Departments of Cell Biology and of Pharmacology and Neurosciences, Georgetown University Medical Center, Washington, D.C. 20057, Unité INSERM U410, Faculté de Médecine Xavier Bichat, 16 Rue Henri Huchard, 75870 Paris Cedex 18, France, and Unité INSERM U472, 16 Avenue Paul Vaillant-Couturier, Batiment INSERM, 94807 Villejuif, France