1. Laboratory of Chromosome Dynamics, Institute of Molecular and Cellular Biosciences, University of Tokyo, Yayoi, Tokyo 113-0032, Japan.
2. Department of Biological Sciences, Graduate School of Science, University of Tokyo, Yayoi, Tokyo 113-0032, Japan.
3. First Department of Pathology, Hamamatsu University School of Medicine, Hamamatsu, Shizuoka 431-3192, Japan.