1. Division of Biology, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.
2. Laboratory of Molecular Immunology, The Rockefeller University, New York, NY 10065, USA.
3. Charite Universitätsmedizin, D-10117 Berlin, Germany.
4. Beth Israel Deaconess Medical Center, Boston, MA 02215, USA.
5. Howard Hughes Medical Institute, The Rockefeller University, New York, NY 10065, USA.
6. Howard Hughes Medical Institute, California Institute of Technology, 1200 East California Boulevard, Pasadena, CA 91125, USA.