1. A. B. Hancock, Jr. Memorial Laboratory for Cancer Research, Departments of Biochemistry, Chemistry, and Pharmacology, Vanderbilt Institute of Chemical Biology, and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, Tennessee 37232, and The Skaggs Institute for Chemical Biology, Departments of Cell Biology and Chemistry, The Scripps Research Institute, La Jolla, California 92037