1. Laboratory of Experimental and Computational Biology, National Cancer Institute-Frederick, NIH, Miller Drive, Frederick, Maryland 21702-1201, Department of Microbiology and Immunology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda, Maryland 20814-4799, and Institute of Human Virology, University of Maryland Biotechnology Institute, University of Maryland, Baltimore, Maryland 21201