1. Laboratory of Gene Function, Kazusa DNA Research Institute 2‐6‐7 Kazusa‐kamatari Kisarazu, Chiba 292‐0818 Japan
2. Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo 7‐3‐1 Hongo Bunkyo‐ku, Tokyo 113‐0033, Japan
3. PRESTO, Japan Science and Technology Agency 4‐1‐8 Honcho Kawaguchi, Saitama 332‐0012 Japan