1 Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan.
2 Division of Virology, Department of Microbiology and Immunology, Institute of Medical Science, University of Tokyo, Tokyo 108-8639, Japan; yoshihiro.kawaoka@wisc.edu.
3 Influenza Research Institute, Department of Pathobiological Sciences, School of Veterinary Sciences, University of Wisconsin-Madison, Madison, WI 53711.
4 Department of Special Pathogens, International Research Center for Infectious Diseases, Institute of Medical Science, University of Tokyo, Minato-ku, Tokyo 108-8639, Japan.