1. Department of Viral Infection, Institute of Medical Science, University of Tokyo, Shirokanedai 4‐6‐1, Minato‐ku, Tokyo 108, Japan,
2. School of Pharmaceutical Sciences, Kitasato University, Shirokane 5‐9‐1, Minato‐ku, Tokyo 108, Japan,
3. Department of Bacteriology, Hiroshima University School of Medicine, Kasumi 1‐2‐3, Minami‐ku, Hiroshima 734, Japan