Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn|Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn|Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn|Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn|Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn|Center for Theoretical Biology, Peking University, Beijing100871, PR China. wangquan@ctb.pku.edu.cn