[1] Burroughs Wellcome Fund, Res Triangle Pk, NC 27709 USA
[2] Harvard Univ, Sch Publ Hlth, Dept Immunol & Infect Dis, Boston, MA 02116 USA
[3] Univ Notre Dame, Dept Biol Sci, Notre Dame, IN 46556 USA
[4] Univ Penn, Penn Ctr Bioinformat, Dept Genet, Philadelphia, PA 19104 USA