1. School of Mathematical Sciences, Peking University 1 , Beijing 100871, China
2. Center for Bioinformatics, National Laboratory of Protein Engineering and Plant Genetic Engineering, School of Life Sciences, Peking University 2 , Beijing 100871, China
3. Courant Institute of Mathematical Sciences, New York University 3 , New York, New York 10003, USA
4. Department of Mathematics, University of California, Berkeley 4 , Berkeley, California 94720, USA
5. Yuanpei College, Peking University 5 , Beijing 100871, China
6. Center for Quantitative Biology, Peking University 6 , Beijing 100871, China