1. Department of Virology and Parasitology, Yamaguchi University School of Medicine, Kogushi, Ube, Yamaguchi 755, Japan
2. Department of Microbiology, Tokyo Medical and Dental University School of Medicine, Yushima, Bunkyo-ku, Tokyo 113, Japan
3. Bristol-Myers Squibb Research Institute, Shimomeguro, Meguro-ku, Tokyo 153, Japan