1. From the Departments of Molecular Physiology and Biophysics (B.M.P., Y.W., N.R.A., D.W.M., M.M.L.), Medicine (T.N., M.M.L.), and Biology (J.R.H.), University of Vermont College of Medicine, University of Vermont, Burlington, Vt; and Department of Genetics (P.G.B., C.E.S., J.G.S.), Howard Hughes Medical Institute and Harvard Medical School, Boston, Mass.