1. Department of Molecular and Experimental Medicine, The Scripps Research Institute, 10550 North Torrey Pines Road, MEM-255, La Jolla, California 92037, Laboratoire de Chimie et Biochimie Pharmacologiques et Toxicologiques, UMR 8601 CNRS, Université Paris V45, Rue des Saints-Pères, 75270 Paris Cedex 06, France, and Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, MB8, La Jolla, California 92037