Center for Computational Biology, College of Biological Science and Technology, Beijing Forestry University, Beijing 100083, China. Tel.: +86-10-6233-6283, Fax: +86-10-6233-6164, rwu@bjfu.edu.cn, Center for Statistical Genetics, The Pennsylvania State University, Hershey, PA 17033, USA. Tel.: +1-717-531-2037, Fax: +1-717-531-0480, E-mail: rwu@phs.psu.edu.