1. Departments of Therapeutic Radiology and Genetics, Yale University, Boyer Center 354, 295 Congress Avenue, New Haven, Connecticut 06536, Laboratoire de Biophysique, Muséum National d'Histoire Naturelle, INSERM U201, CNRS UA481, 43, rue Cuvier, 75005 Paris, France, Food and Drug Administration, 200 C Street, SW, Washington, DC, 20204, National Institute on Aging, NIH, 5600 Nathan Shock Drive, Baltimore, Maryland 21224-6825, and Gilead Sciences, 353 Lakeside Drive, Foster City, California 94404