1. School of Life Sciences, Tsinghua University, Beijing 100084, China
2. Institute for Interdisciplinary Information Sciences, Tsinghua University, Beijing 100084, China
3. Present address: Language Technologies Institute, Carnegie Mellon University, Pittsburgh, PA 15232, USA
4. Department of Computer Science and Engineering, University of California, Riverside, CA 92521, USA
5. MOE Key Lab of Bioinformatics and Bioinformatics Division, TNLIST/Department of Computer Science and Technology, Tsinghua University, Beijing 100084, China