1. Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639
2. CREST, Japan Science and Technology Corporation, Saitama 332-0012
3. Graduate School of Pharmaceutical Sciences, University of Tokyo, Tokyo 113-0033, Japan
4. Department of Pathobiological Sciences, University of Wisconsin, Madison, Wisconsin 53706
5. Special Pathogens Program, National Microbiology Laboratory, Canadian Science Centre for Human and Animal Health, Winnipeg, Manitoba R3E 3R2, Canada