[1] HARVARD UNIV, SCH MED,DANA FARBER CANC INST,DEPT PATHOL, DIV HUMAN RETROVIROL, BOSTON, MA 02115 USA
[2] HARVARD UNIV, SCH PUBL HLTH, DEPT CANC BIOL, BOSTON, MA 02115 USA
[3] ST GEORGE HOSP, SCH MED, DEPT CELLULAR & MOLEC SCI, DIV COMMUNICABLE DIS, LONDON SW17 0RE, ENGLAND