1. Department of Pediatrics; Teikyo University School of Medicine; Itabashi-ku Tokyo 173-8605 Japan
2. Department of Developmental Medical Sciences, Graduate School of Medicine; The University of Tokyo; Bunkyo-ku Tokyo 113-0033 Japan
3. Division of Virology; Kagawa Prefectural Institute of Health; Kagawa 760-0065 Japan
4. Department of Pediatrics; Sapporo Medical University; Sapporo Hokkaido 060-8556 Japan