1. Division of HIV/AIDS Prevention, National Center for HIV, Hepatitis, STD, and Prevention, Centers for Disease Control and Prevention, 1600 Clifton Road, Atlanta, GA 30329, USA.
2. Division of Laboratory Sciences, National Center for Environmental Health, Centers for Disease Control and Prevention, Atlanta, GA 30329, USA.
3. Southwest National Primate Research Center, Southwest Foundation for Biomedical Research, San Antonio, TX 78245, USA.