Kenji Itao1, Kunihiko Kaneko2,3. 1. Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan. 2. Department of Basic Science, Graduate School of Arts and Sciences, University of Tokyo, Meguro-ku, Tokyo 153-8902, Japan; kaneko@complex.c.u-tokyo.ac.jp. 3. Center for Complex Systems Biology, Universal Biology Institute, University of Tokyo, Tokyo 113-0033, Japan.