1. Medical Research Council, Cancer Cell Unit, Hutchison MRC Research Centre, Hills Road, Cambridge CB2 2XZ, United Kingdom, Seattle Genetics, Inc., 21823 30th Drive Southeast, Bothell, Washington 98021, and The Pennsylvania State University, College of Medicine, Department of Medicine, H039, 500 University Drive, P.O. Box 850, Hershey, Pennsylvania 17033-0850