Affiliation: | 1 Division of Molecular Structure, National Institute for Medical Research, the Ridgeway, Mill Hill, London NW7 1AA, UK 2 Institute of Comparative Medicine, University of Glasgow Veterinary School, Glasgow, UK 3 Division of Mathematical Biology, National Institute for Medical Research, the Ridgeway, Mill Hill, London NW7 1AA, UK 4 Division of Virology, National Institute for Medical Research, the Ridgeway, Mill Hill, London NW7 1AA, UK |