1. Department of Biochemistry and the Siebens-Drake Medical Research Institute, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, N6A 5C1, Canada, The University of Texas M.D. Anderson Cancer Center, Science Park-Research Division, P.O. Box 389, Smithville, Texas 78957, United States, and The No.3 Department, Institute of Chemical Defense, P.O. Box 1048, Beijing 102205, China