1. Department of Microbiology and Immunology, University of California , San Francisco, California , USA
2. Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford University , Stanford, California , USA
3. Department of Genetics, Stanford University School of Medicine, Stanford University , Stanford, California , USA
4. National Institute for Biological Standards and Control , South Mimms, Hertfordshire , United Kingdom
5. Department of Microbiology and Immunology, Vagelos College of Physicians and Surgeons, Columbia University , New York, New York , USA
6. Center for Biologics Evaluation and Research, US Food and Drug Administration , Silver Spring, Maryland , USA