1. Mallinckrodt Institute of Radiology, Washington University Medical School, Box 8225, 510 S. Kingshighway Blvd., St. Louis, Missouri 63110, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556-5670, Howard Hughes Medical Institute, and Departments of Medicine and Molecular Microbiology, Washington University Medical School, St. Louis, Missouri 63110