1. Beckman Laboratory for Structural Biology, Department of Structural Biology, Stanford School of Medicine, Stanford, California 94305
2. Protein Structure Group, National Institute for Medical Research, The Ridgeway, Mill Hill, London NW7 1AA, U.K.
3. Department of Structural Biology, Weizmann Institute of Science, Rehovot, 7600, Israel
4. Department of Medicinal Chemistry, University of Washington, Seattle, Washington 98195-7610