1. Department of Molecular Medicine, Mayo Clinic, Rochester, MN 55905.
2. Genetic Immunotherapy Section, Laboratory of Host Defenses, National Institute of Allergy and Infectious Diseases, National Institute of Health, Bethesda, MD 20892.
3. Division of Infection and Immunity, Wohl Virion Centre, Windeyer Institute, University College London, London W1T 4JF, UK.