1. Division of Pathology; National Institute of Health Sciences; 1-18-1 Kamiyoga, Setagaya-Ku Tokyo 158-8501 Japan
2. Central Animal Division; National Cancer Center Research Institute; 5-1-1 Tsukiji, Chuo-ku Tokyo 104-0045 Japan
3. Biological Safety Research Center; National Institute of Health Sciences; 1-18-1 Kamiyoga, Setagaya-Ku Tokyo 158-8501 Japan