1. Department of Biology, University of Washington, Seattle, WA, 98195, USA
2. Department of Mathematics and Computer Science, Muhlenberg College, Allentown, PA, 18104, USA
3. Howard Hughes Medical Institute, Fred Hutchison Cancer Research Center, Departments of Genome Sciences and Statistics, University of Washington, Seattle, WA, 98109, USA
4. Department of Statistics, University of California, Irvine, Irvine, CA, 92697, USA