Affiliation: | a Howard Hughes Medical Institute and Department of Microbiology and Immunology, University of California San Francisco, HSE 401, 513 Parnassus Avenue, San Francisco, CA 94143-0414, USA b J.P. Robarts Research Institute and the Department of Microbiology and Immunology, University of Western Ontario, Rm. 133 SDRI, 1400 Western Road, London, Ontario, Canada, N6G 2V4 |