1. Section of Immunobiology, Yale University School of Medicine, New Haven, CT 06510,
2. Program in Molecular Medicine, Department of Biochemistry and Molecular Biology, University of Massachusetts Medical School, and Howard Hughes Medical Institute, Worcester, MA 01605,
3. Howard Hughes Medical Institute, Section of Immunobiology, Yale University School of Medicine, P.O. Box 208011, New Haven, CT 06520‐8011, U.S.A.