1. Cancer Prevention Division, National Cancer Center Research Institute, Tsukiji 5—1—1, Chuo-ku, Tokyo 104, Japan
2. Current address: Ehime University School of Medicine, Ehime 791—02, Japan
3. Faculty of Pharmaceutical Sciences, Okayama University, Tsushima, Okayama 700, Japan
4. Faculty of Medicine, Gifu University School of Medicine, Tsukasa-machi, Gifu 500, Japan
5. Department of Environmental Medicine, New York University Medical Center, New York, NY 10016