When the NA gene from H9N2, H5N1, or A(H1N1)pdm09 virus was expressed on a PR8 background, the replication kinetics were similar in vitro (MDCK cells) and in vivo (mice) (Chen et al., 2013), yet the initial infection kinetics and virus-induced fusion and elution from erythrocytes were affected, implicating a role for NA during the early stage of infection (Chen et al., 2013). This evidence concerns the gene XK and infection.