1. Department of Genetics, Harvard Medical School, and Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts 02114 (E.D., B.G.R., N.A.A., F.M.A.);
2. Whitehead Institute for Biomedical Research, 9 Cambridge Center, Cambridge, Massachusetts 02142 (S.R.);
3. Harvard College, Cambridge, Massachusetts 02138 (L.M.S.); and Departments of
4. Biochemistry (M.M., R.J.C., P.J.O., R.W.D) and
5. Genetics (R.W.D), Stanford University School of Medicine, Stanford, California 94305