1. O. Livnah, E. A. Stura, and I. A. Wilson are in the Department of Molecular Biology and at the Skaggs Institute of Chemical Biology, The Scripps Research Institute, 10666 North Torrey Pines Road, La Jolla, CA 92037, USA.
2. D. L. Johnson, S. A. Middleton, L. S. Mulcahy, and L. K. Jolliffe are at the R. W. Johnson Pharmaceutical Research Institute, Drug Discovery Research, 1000 Route 202, Box 300, Raritan, NJ 08869, USA.
3. N. C. Wrighton and W. J. Dower are at Affymax Research Institute, 4001 Miranda Avenue, Palo Alto, CA 94304, USA.