1. Consejo Superior de Investigaciones Cientificas, Centro de Investigaciones Biologicas, Madrid, Spain 28006
2. Department of Medicinal Chemistry, Merck Sharp and Dohme Research Laboratories, West Point, Pennsylvania 19486
3. Department of Biochemistry and Molecular Biology, Merck Institute for Therapeutic Research, Merck Sharp and Dohme Research Laboratories, Rahway, New Jersey 07065