1. Department of Molecular Virology and Microbiology, Baylor College of Medicine, Houston, Texas 77030
2. United States Army Medical Research Insititute of Infectious Diseases, 1425 Porter Street, Fort Detrick, Maryland 21702
3. Department of Gastrointestinal Sciences, Christian Medical College and Hospital, Vellore 632 004, India
4. Veterans Affairs Medical Center, 2002 Holcombe Boulevard, Houston, Texas 77030