1. Department of Mathematical Sciences, University of Texas at Dallas, Richardson, Texas 75080, USA
2. Department of Applied Mathematics, University of Washington, Seattle, Washington 98195, USA
3. Department of Biological Sciences, Center for Systems Biology, University of Texas at Dallas, Richardson, Texas 75080, USA
4. MOE Key Lab and Division of Bioinformatics, CSSB, TNLIST, Tsinghua University, Beijing 100084, China