1. Department of Molecular and Cellular Biology, Institute for Frontier Medical Sciences, Kyoto University Kyoto 606‐8397 Japan
2. Core Research for Evolutional Science and Technology, JST Sapporo 060‐8556 Japan
3. Department of Biochemistry, Sapporo Medical University School of Medicine Sapporo 060‐8556 Japan
4. McGill Cancer Centre, McGill University Montréal H3G 1Y6 Québec Canada