1. Laboratory of Public Health, Department of Environmental Veterinary Sciences; Graduate School of Veterinary Medicine, Hokkaido University; Sapporo Hokkaido 060-0818 Japan
2. Division of Cell Biology; Cancer Institute, Hokkaido University School of Medicine; Sapporo Hokkaido 060-8638 Japan
3. First Department of Surgery; Okayama University; Okayama Okayama 700-8558 Japan