1. Divisions of Basic Sciences and Clinical Research, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, Washington 98109, Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Disease, National Institutes of Health, Bethesda, Maryland 20892, and Department of Protein Biochemistry, ICOS Corporation, 22021 20th Avenue, Southeast, Bothell, Washington 98021