1. Boston University School of Medicine, Department of Pharmacology and Experimental Therapeutics, Boston, MA 02118.
2. M.L.H., Z.F.M., and S.S. were equivalent contributors and thus co-authors of this study.
3. Spring Arbor University, Biochemistry Department, Spring Arbor, MI 49283.
4. Present address: Molecular Oncology Research Institute, Tufts–New England Medical Center, 75 Kneeland Street, 12th Fl., Tufts-NEMC # 5609, Boston MA 02111.
5. School of Biological and Biomedical Sciences, University of Durham, Durham, DH1 3LE, UK.
6. Safety Assessment, Department of Molecular and Investigative Toxicology, Merck Research Laboratories, West Point, PA 19486.