[1] Inst Canc Res, Chester Beatty Labs, Sect Virol, London SW3 6JB, England
[2] Imperial Coll Sch Med, Jefferiss Res Trust Labs, Dept Genitourinary Med, London WL 1NY, England
[3] Serono Pharmaceut Res Inst, CH-1228 Geneva, Switzerland
[4] Katholieke Univ Leuven, Rega Inst Med Res, B-3000 Louvain, Belgium