1. Laboratory of Molecular Pharmacology, Division of Basic Sciences, National Cancer Institute, Bethesda, Maryland 20892-4255, Southern Research Institute, 431 Aviation Way, Frederick, Maryland 21701, Achillion Pharmaceuticals, 1003 West 7th Street, Suite 401, Frederick, Maryland 21701, and Dipartimento Farmaco Chimico Tecnologico, Universitá di Siena, Banchi di Sotto, 55, 53100 Siena, Italy