1. Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, Texas 77030
2. Laboratory of Infectious Diseases, NIAID, National Institutes of Health, Bethesda, Maryland 20892
3. Department of Biochemistry and Molecular Biology, University of Texas, M. D. Anderson Cancer Center, Houston, Texas 77030