1. Department of Immunology, University of Washington, Seattle, WA 98195, USA.
2. Department of Developmental Biology, Stanford University School of Medicine, Stanford, CA 94305, USA.
3. Molecular and Cellular Biology Program, University of Washington, Seattle, WA 98195, USA.
4. Department of Pediatrics, University of Washington, Seattle, WA 98195, USA.
5. Departments of Genome Sciences, Microbiology, and Medicine, University of Washington, Seattle, WA 98195, USA.