1. Contribution from the Departments of Medical Genetics and Biochemistry, University of Toronto, Toronto, Ontario, Canada, M5S 1A8, Department of Chemistry, University of Toronto, Toronto, Ontario, Canada, M5S 1A8, Department of Pharmaceutical Sciences, Leslie Dan Faculty of Pharmacy, University of Toronto, Toronto, Ontario, Canada, M5S 2S2, and National Cancer Institute, Laboratory of Biochemistry, Building 37, Bethesda, Maryland 20892