1. University Chemical Laboratory, University of Cambridge, Cambridge, U.K., GlaxoWellcome, Medicines Research Centre, Gunnels Wood Road, Stevenage SG1 2NY, U.K., GlaxoWellcome, Chemical Development, Dartford, Kent, U.K., Roche Bioscience, Palo Alto, California, and The Wolfson Institute for Biomedical Research, University College London, The Cruciform Building, Gower Street, London WC1E 6AU, U.K.