1. Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, The Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Institute of Biotechnology, Cambridge University, Tennis Court Road, Cambridge CB2 1QT, U.K.