When the NA genes from influenza H9N2, H5N1 or A(H1N1) pdm09 virus was respectively expressed on a PR8 background, the replication kinetics, both in vitro and in vivo, were not affected, but different effects on infection initiation, virus release and fusion of infected cells were observed, implicating a role for NA during the early stage of infection [78]. This evidence concerns the gene XK and infection.