1. Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, University of Colorado Cancer Center, Aurora, Colorado 80045, Dana−Farber Cancer Institute, 44 Binney Street, Boston, Massachusetts 02115, Department of Cellular and Molecular Pharmacology, University of California—San Francisco, San Francisco, California 94158, Howard Hughes Medical Institute, Chemistry & Chemical Biology, Harvard University, Broad Institute of Harvard and MIT, Cambridge, Massachusetts 02142