It is believed that the binding of HAs (binding avidity and ligand specificity) to host cells has to match the activity of NAs (rate of activity and substrate specificity) for the influenza virus to achieve efficient infection and replication.27,29 It would be interesting to investigate whether the relatively high activity of N1av and N1pdm correlated with stronger HA-receptor avidity; and the interplay between HA and NA on the same viral surface also needs to be considered. The gene discussed is XK; the disease is infection.