1. Department of Biochemistry and Biophysics and UNC Lineberger Comprehensive Cancer Center, University of North Carolina School of Medicine, Chapel Hill, North Carolina 27599, Department of Biochemistry and Biophysics and Johnson Research Foundation, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104, Glaxo SmithKline Inc., Research Triangle Park, North Carolina 27713, Cognosci Inc., Research Triangle Park, North Carolina 27709, and BioZyme, Apex, North Carolina 27523