1. D. H. Fremont, Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032, USA, and Howard Hughes Medical Institute, Division of Basic Immunology, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO 80206, USA.
2. W. A. Hendrickson, Howard Hughes Medical Institute, Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032, USA.
3. P. Marrack and J. Kappler, Howard Hughes Medical Institute, Division of Basic Immunology, National Jewish Center for Immunology and Respiratory Medicine, Denver, CO 80206, USA.