1. Gladstone Institute of Virology and Immunology, University of California, San Francisco, San Francisco, California 94141 present address: SyStemix, Inc. Palo Alto, California 94303
2. Gladstone Institute of Virology and Immunology, University of California, San Francisco, San Francisco, California 94141
3. Gladstone Institute of Virology and Immunology, University of California, San Francisco, San Francisco, California 94141. 2Present address: SyStemix, Inc., Palo Alto, California 94303; Department of Microbiology and Immunology and Department of Medicine, University of California, San Francisco, San Francisco, California 94103