1. Howard Hughes Medical Institute and the Department of Microbiology and Immunology, Stanford University, Stanford, California 94305-5323;, , , ,
2. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139;
3. Genentech, Incorporated, South San Francisco, California 94080-4990;
4. Department of Microbiology and Immunology, University of California, San Francisco, California 94143;