1. Department of Molecular Biology; Okayama University Graduate School of Medicine, Dentistry, and Pharmaceutical Sciences; Okayama Okayama 700-8558 Japan
2. Department of Virology and Preventive Medicine; Gunma University Graduate School of Medicine; Maebashi Gunma 371-8511 Japan
3. Gastroenterological Center; Yokohama City University of Medical Center; Yokohama Kanagawa 236-0004 Japan