1. Vaccine Research Center, National Institute of Allergy and Infectious Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
2. Organic Chemistry Section, M. D. Anderson Cancer Center, University of Texas, Houston, TX 77030, USA.
3. Department of Virology and Immunology, Texas Biomedical Research Institute, San Antonio, TX 78227, USA.
4. Department of Pathology and Laboratory Medicine, Emory University, Atlanta, GA 30322, USA.