1. Department of Immunology and Molecular Biology, Scripps Research Institute, La Jolla, CA 92037, USA.
2. R. W. Johnson Pharmaceutical Research Institute, San Diego, CA 92121, USA.
3. Viral and Rickettsial Disease Laboratory, California Department of Health Services, Berkeley, CA 94704, USA.
4. Laboratory of Tumor Cell Biology, Virus Biology Section, National Cancer Institute-Frederick Cancer Research and Development Center, Frederick, MD 21702, USA.
5. Department of Pediatrics, University of California at Los Angeles School of Medicine, Los Angeles, CA 90024, USA.
6. Aaron Diamond AIDS Research Center, New York University School of Medicine, New York, NY 10016, USA.