1. Molecular Pharmacology Department, St. Jude Children's Research Hospital, Memphis, Tennessee 38105, Department of Biochemistry and Molecular Biology, University of Southern California Medical School, Los Angeles, California 90033, DuPont Merck Pharmaceutical Co., Experimental Station, Wilmington, Delaware 19880-0336, and Departments of Biochemistry and Medicine, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146