1. From the Laboratory of Pathology (J.S.I., M.A.A., M.T., D.D.R.) and Radiation Biology Branch (F.H., M.C.K.), Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Md; and the Department of Biochemistry and Molecular Biophysics (L.K.P., W.A.F.), Washington University School of Medicine, St. Louis, Mo.