[1] HARVARD UNIV, DEPT BIOCHEM & MOLEC BIOL, CAMBRIDGE, MA 02138 USA
[2] HARVARD UNIV, SCH MED, DEPT MICROBIOL & MOLEC GENET, BOSTON, MA 02115 USA
[3] HARVARD UNIV, SCH MED, SHIPLEY INST MED, BOSTON, MA 02115 USA
[4] BRIGHAM & WOMENS HOSP, DEPT MED, BOSTON, MA 02115 USA
[5] HARVARD UNIV, HOWARD HUGHES MED INST, CAMBRIDGE, MA 02138 USA