1. MRC Clinical Science Centre, Imperial College School of Medicine, Hammersmith Hospital, London W12 ONN,1
2. Division of Virology, Department of Pathology, Cambridge University, Cambridge CB2 1QP,2
3. Department of Biology, Imperial College of Science Technology and Medicine, South Kensington, London SW7 2AZ,3 and
4. QT Genetics Ltd., Cambridge CB4 3GA,4 United Kingdom