Department of Genome Sciences and Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA. troyz@u.washington.edu|Department of Genome Sciences and Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA. troyz@u.washington.edu|Department of Genome Sciences and Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA. troyz@u.washington.edu|Department of Genome Sciences and Howard Hughes Medical Institute, University of Washington, Seattle, WA 98195, USA. troyz@u.washington.edu