Department of Statistics, Purdue University, West Lafayette, IN, Department of Genome Sciences, University of Washington, Seattle, WA 98195 and Department of Computer Science, Purdue University, West Lafayette, IN 47907, USA Department of Statistics, Purdue University, West Lafayette, IN, Department of Genome Sciences, University of Washington, Seattle, WA 98195 and Department of Computer Science, Purdue University, West Lafayette, IN 47907, USA.