1. Departments of Molecular Biology and Chemistry and The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Departments of Pediatrics and Pharmacology, Yale University School of Medicine, 3094 LMP, 333 Cedar Street, P.O. Box 208064, New Haven, Connecticut 06520-8064