1. Laboratory of Cell Biology, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892, Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901, Center for Biologics Evaluation and Research, Food and Drug Administration, Rockville, Maryland 20852 and Pathology Section, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892