[1] HARVARD UNIV, SCH MED, SIDNEY FARBER CANC INST, BOSTON, MA 02115 USA
[2] HARVARD UNIV, SCH MED, DEPT MICROBIOL & MOLEC GENET, BOSTON, MA 02115 USA
[3] BAYLOR COLL MED, DEPT VIROL & EPIDEMIOL, HOUSTON, TX 77030 USA
[4] UNIV TENNESSEE, DEPT MICROBIOL, KNOXVILLE, TN 37916 USA