1. J. D. Altman and M. G. McHeyzer-Williams, Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305-5428, USA.
2. P. A. H. Moss, P. J. R. Goulder, D. H. Barouch, J. I. Bell, A. J. McMichael, Institute for Molecular Medicine, John Radcliffe Hospital, Headington, Oxford OX3 9DU, UK.
3. M. M. Davis, The Howard Hughes Medical Institute, Department of Microbiology and Immunology, Beckman Center, Room B221, Stanford University, Stanford, CA 94305-5428, USA.