1. Department of Molecular Biology, Osaka Bioscience Institute Suita Osaka 565‐0874 Japan
2. School of Biostudies, Kyoto University, Sakyoku Kyoto 606‐8502 Japan
3. Department of Molecular Oncology, Osaka Bioscience Institute Suita Osaka 565‐0874 Japan
4. Laboratory of Developmental Neurogenetics, The Rockefeller University 1230 York Avenue New York NY 10021‐6399 USA