1. Graduate Program in Biomedical Sciences, University of California, San Diego, California 92093, Department of Molecular, Cellular, & Developmental Biology, Interdepartmental Program in Biochemistry and Molecular Biology, University of California, Santa Barbara, California 93106, Department of Pharmacology, University of Texas Southwestern Medical Center, Dallas, Texas 75235, and Department of Chemistry & Biochemistry, Howard Hughes Medical Institute, University of Colorado, Boulder, Colorado 80303