[1] Harvard Univ, Sch Med, Howard Hughes Med Inst, Dept Genet, Boston, MA 02115 USA
[2] Harvard Univ, Sch Med, Dept Microbiol & Mol Genet, Boston, MA 02115 USA
[3] Johns Hopkins Univ, Dept Biostat, Baltimore, MD 21205 USA
[4] Faulkner Hosp, Dept Pathol, Jamaica Plain, MA USA