1. Deptartment of Developmental Neurobiology, St Jude Children's Research Hospital Memphis Tennessee 38105 USA
2. Advanced Science Research Center, Kanazawa University Kanazawa 920‐8640 Japan
3. PRESTO, Japan Science and Technology Agency Saitama 332‐0012 Japan
4. Department of Anatomy, Hokkaido University School of Medicine Sapporo 060‐8638 Japan
5. Department of Physiology, Keio University School of Medicine Tokyo 160‐8582 Japan
6. Department of Molecular Neurobiology, School of Medicine, University of Tokyo Tokyo 113‐0033 Japan