The reference influenza virus strains A/PR8/34 (H1N1) (abbreviated PR8), B/hubeiwujiagang/158/2009 (Yamagata lineage, abbreviated BY) and B/heilongjianghulan/116/2010 (Vic-toria lineage, abbreviated BV) were kindly provided by the Beijing Center for Disease Control and Prevention. The viruses were grown in MDCK cells as described elsewhere [5] . The titers of virus strains were determined by hemagglutination tests and expressed as hemagglutinating units (HAU). For Western blot analysis, the inactivated viruses were lyzed in a lysis buffer (50 mM Tris-HCl, 150 mM NaCl, 5 mM EDTA, 1% Triton-X, pH 7.4) supplemented with a protease inhibitor cocktail (Roche, Indianapolis, IN).