XK and infection: HA and NA are both glycoproteins embedded in influenza viral envelope and play critical but distinct roles in the virus replication cycle: HA initiates infection by binding to host cell SA receptors and docking fusion with endosome membrane to trigger replication, and NA sustains infection by cleaving SA from the host cell surface to release the nascent virions for new replication cycles and cleaving the SA from viral envelope to keep viruses from aggregation (19, 20).