1. Department of Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, MB-4, La Jolla, California 92037, Department of Immunology, Lerner Research Institute, The Cleveland Clinic, 9500 Euclid Avenue, Cleveland, Ohio 44195, AstraZeneca Structural Chemistry Laboratories, Pepparedsleden 1, 431 83 Mölndal, Sweden, and AstraZeneca R&D Charnwood, Bakewell Road, Loughborough, Leicestershire LE11 5RH, U.K.