Philip E. Bourne*1, Helen M. Berman2, Brian McMahon3, Keith D. Watenpaugh4, John D. Westbrook2 and Paula M.D. Fitzgerald5 * To whom correspondence should be addressed. 1 San Diego Supercomputer Center 4 Physical and Analytical Chemistry 10100 Johns Hopkins Drive Pharmacia and Upjohn La Jolla San Diego CA 92037 USA 7255-209-102 & Department of Pharmacology 301 Henrietta Street University of California San Diego Kalamazoo MI 49001 USA San Diego CA 92093 USA 2 Department of Chemistry 5 Merck Research Laboratories Rutgers University PO Box 2000 Ry50-105 PO Box 939 Rahway NJ 07065 USA Piscataway NJ 08855 USA 3 The International Union of Crystallography 5 Abbey Square Chester CH1 2HU UK