[1] Hammersmith Hosp, Imperial Coll, MRC,Clin Sci Ctr, Sch Med,Gene Targeting Grp, London W12 0NN, England
[2] Univ Cambridge, Dept Pathol, Div Virol, Cambridge CB2 1QP, England
[3] Univ London Imperial Coll Sci Technol & Med, Dept Biol, London SW7 2AZ, England
[4] QT Genet Ltd, Cambridge CB4 3GA, England