1. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138 (J.C., L.R.R., P.D.K., N.E.P.);
2. Department of Genetics, Harvard Medical School, Boston, Massachusetts 02115 (G.J., F.M.A.); and
3. Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts 02114 (G.J., F.M.A.)