1. Department of Chemistry and Biochemistry, University of California, Santa Cruz, California 95064, Department of Genetics, Stanford University School of Medicine, Palo Alto, California 94305, Stanford Genome Technology Center, Palo Alto, California 94304, Faculty of Pharmacy, Donnelly Centre for Cellular and Biomolecular Research, University of Toronto, 168 College Street, Toronto, Ontario, M5S3E1, Canada, and Department of Biochemistry, Stanford University School of Medicine, Palo Alto, California