1. Computational Biology Program, Fred Hutchinson Cancer Research Center, Seattle, Washington, USA.
2. Department of Genome Sciences, and University of Washington, Seattle, Washington, USA.
3. Department of Statistics, University of Washington, Seattle, Washington, USA.
4. Howard Hughes Medical Institute, Chevy Chase, Maryland, USA.
5. Departments of Biology and Mathematics, Institute of Ecology and Evolution, University of Oregon, Eugene, Oregon, USA.