Department of Computational Biology, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan. thanet@cb.k.u-tokyo.ac.jp|Department of Computational Biology, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan. thanet@cb.k.u-tokyo.ac.jp|Department of Computational Biology, The University of Tokyo, Kashiwa, Chiba 277-8568, Japan. thanet@cb.k.u-tokyo.ac.jp