1. Macfarlane Burnet Centre for Medical Research, Fairfield (Melbourne), Victoria, Australia 3078
2. New England Regional Primate Center/Harvard Medical School, Boston, Massachusetts 02115
3. Health Canada, National Laboratory of HIV Pathogenesis, Ottawa, Ontario K1A 0L2, Canada
4. AIDS Collaborating Centre, National Institute of Biological Standards and Control, Potters Bar EN6 3QG, England