1. Division of Comparative Medicine, Medical Research Council Clinical Research Centre, Watford Road, Harrow, Middlesex HA1 3UJ, United Kingdom; Department of Medical Physics, Royal Postgraduate Medical School, Hammersmith Hospital, Du Cane Road,
London W12 0HS, United Kingdom
2. Comparative Biology Centre, University of Newcastle upon Tyne, Framlington Place, Newcastle upon Tyne NE2 4HH, United Kingdom