1. Biological Chemistry, Division of Biomedical Sciences, Imperial College of Science, Technology and Medicine, Sir Alexander Fleming Building, South Kensington, London, SW7 2AZ, U.K., Physical Sciences Research Unit, Glaxo Smithkline Medicines Research Centre, Gunnels Wood Road, Stevenage, Herts, SG1 2NY, U.K., and Preclinical Safety Sciences, Glaxo Smithkline Research and Development, Park Road, Ware, Herts., SG12 0DP, U.K.