1. I. S. Grewal, K. D. Grewal, J. Xu, C. A. Janeway Jr., R. A. Flavell, Howard Hughes Medical Institute, and Section of Immunobiology, Yale University School of Medicine, New Haven, CT 06510, USA.
2. H. G. Foellmer, Yale Neurotransplant Program, Yale University School of Medicine, New Haven, CT 06510, USA.
3. F. Hardardottir and J. L. Baron, Section of Immunobiology, Yale University School of Medicine, New Haven, CT 06510, USA.