1. National Institute of Child Health and Human Development1 and
2. National Institute of Diabetes, Digestive and Kidney Diseases,2National Institutes of Health, and
3. Center for Biologics Evaluation and Research,3 Bethesda, Maryland;
4. Walter Reed Army Institute For Research, Washington, D.C.4; and
5. Department of Molecular Microbiology and Immunology, University of Missouri, Columbia, Missouri5