1. Laboratory of Chromosome Dynamics, Institute of Molecular and Cellular Biosciences, and Graduate School of Agricultural and Life Science, University of Tokyo, Tokyo 113-0032, Japan
2. Institute of Developmental Biology and Molecular Medicine and School of Life Sciences, Fudan University, Shanghai 200433, China
3. Howard Hughes Medical Institute and Department of Molecular, Cellular, and Developmental Biology, University of Colorado, Boulder, CO 80309