1. Howard Hughes Medical Institute, Basic Sciences Division, Fred Hutchinson Cancer Research Center, 1100 Fairview Avenue North, Seattle, WA 98109, USA.
2. Department of Genome Sciences, University of Washington, Seattle, WA 98115, USA.
3. Molecular and Cellular Biology Program, University of Washington, Seattle, WA 98115, USA.
4. Departments of Mathematics, Molecular and Cell Biology, and Electrical Engineering and Computer Sciences, University of California–Berkeley, Berkeley, CA 94720, USA.