1. Central Animal Division, National Cancer Center Research Institute, 5-1-1 Tsukiji, Chuo-ku, Tokyo 104-0045 Japan
2. Laboratory of Veterinary Pathology, Joint Department of Veterinary Medicine, Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan
3. Department of Molecular Carcinogenesis, Chiba Cancer Center Research Institute, 666-2 Nitonacho, Chuo-ku, Chiba 260-8717, Japan