[1] NCI,FREDERICK CANC RES & DEV CTR,ABL BASIC RES PROGRAM,FREDERICK,MD 21702
[2] UNIV PENN,DEPT MICROBIOL,PHILADELPHIA,PA 19104
[3] GLADSTONE INST VIROL & IMMUNOL,SAN FRANCISCO,CA 94141
[4] UNIV SAN FRANCISCO,SCH MED,DEPT MICROBIOL & IMMUNOL,SAN FRANCISCO,CA 94141