To evaluate the subtype-specificity of epitopes in P5 and P31 further, additional HA proteins of three epidemic human strains from different years (1918, 1934 and 1977) as well as a swine strain were expressed by pCAGGS vector in 293T cells. The reactivity of anti-P5 and P31 sera with the cell lysates was determined by Western blot analysis. Our results showed that the anti-P5 serum strongly reacted with all of the six H1 HA proteins in a manner similar to an antibody against the HA1 of H1N1pdm virus (Fig. 3B ). We found that the anti-P31 sera reactivity was weak against HA proteins from the 1918 and 1977 virus strains (Fig. 3B ). These data indicated that the epitopes in P5 and P31 peptides are relatively conserved among H1-subtype IAVs though these viruses have circulated in the world for almost a century.