1. Roche Products Ltd., Hertfordshire, AL7 3AY, United Kingdom.
2. Division of Virology, Department of Medical Microbiology, St. Mary's Hospital Medical School, London WC2 1PG, United Kingdom.
3. Department of Biochemistry, University of Wales College of Cardiff, Cardiff CF1 1ST, United Kingdom.
4. Medical Research Council Collaborative Centre, London NW7 1AD, United Kingdom.