1. Howard Hughes Medical Institute, University of California, San Francisco, San Francisco, CA 94143, USA.
2. Department of Microbiology and Immunology, University of California, San Francisco, San Francisco, CA 94143, USA.
3. Medical Scientist Training Program, University of California, San Francisco, San Francisco, CA 94143, USA.
4. J. David Gladstone Institutes, San Francisco, CA 94158, USA.
5. Department of Medicine, University of California, San Francisco, San Francisco, CA 94143, USA.