1. P. W. Dempsey, M. E. D. Allison, D. T. Fearon, Wellcome Trust Immunology Unit, Department of Medicine, University of Cambridge School of Medicine, Cambridge CB2 2SP, UK.
2. S. Akkaraju, Program in Immunology, Stanford University School of Medicine, Stanford, CA 94305-5428, USA.
3. C. C. Goodnow, Howard Hughes Medical Institute, Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305-5428, USA.