Department of Biostatistics, Department of Genetics, Department of Computer Science, University of North Carolina, Chapel Hill, NC 27599 USA, Merck Research Laboratories, North Wales, PA, USA, School of Statistics, Renmin University of China, Beijing, Peoples Republic of China, Department of Statistics, North Carolina State University, Raleigh, NC, 27607 USA, Department of Biostatistics, St. Jude Childrens Research Hospital, Memphis, TN 38105, USA and Broad Institute of MIT and Harvard, Cambridge, MA 02141, USA.