1. Department of Biochemistry, University of Washington, Seattle, WA 98195, USA.
2. Department of Microbiology and Immunology, Uniformed Services University, Bethesda, MD 20814, USA.
3. Henry M. Jackson Foundation for the Advancement of Military Medicine, Bethesda, MD 20814, USA.
4. U.S. Public Health Services Commissioned Corps, Rockville, MD 20852, USA.
5. Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA.