1. Centre for Veterinary Science, University of Cambridge, Cambridge CB3 OES,1
2. Department of Biochemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AZ,2 and
3. School of Microbiological, Immunological and Virological Sciences, The Medical School, University of Newcastle, Newcastle upon Tyne NE2 4HH,3 United Kingdom