1. Departments of Biochemistry and Chemistry, 419 Roger Adams Laboratory, University of Illinois at Urbana-Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, Maxygen, Inc., 515 Galveston Drive, Redwood City, California 94063, and Departments of Biopharmaceutical Sciences and Pharmaceutical Chemistry, University of California, San Francisco, California 94143