1. Institute for Systems Biology, Seattle, Washington98109, United States
2. Department of Biochemistry, University of Washington, Seattle, Washington98195, United States
3. Department of Biochemistry and Biophysics, University of California, San Francisco, San Francisco, California94143, United States
4. Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon97403, United States
5. Department of Genome Sciences, University of Washington, Seattle, Washington98195, United States