1. Pharmasset Inc., 1860 Montreal Road, Tucker, Georgia 30084, Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, 635 Barnhil Drive, Indianapolis, Indiana 46202, Laboratory of Molecular Genetics, National Institute of Aging, NIH, 5600 Nathan Shock Drive, Baltimore, Maryland 21224, and Department of Biochemistry and Biophysics, University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, New York 14642