1. Infectious Diseases, Merck Research Laboratories, Kenilworth, NJ 07033, USA.
2. Global Structural Chemistry, Merck Research Laboratories, West Point, PA 19486, USA.
3. Merck Frosst Canada, Kirkland, Quebec H9H 3L1, Canada.
4. Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, 2781-901 Oeiras, Portugal.
5. Screening and Protein Science, Merck Research Laboratories, West Point, PA 19486, USA.
6. Department of Molecular Genetics, University of Toronto, Toronto, Ontario M5S 3E1, Canada.