Neuraminidase (NA) and hemagglutinin (HA) are major membrane glycoproteins found on the surface of influenza virus. HA is a lectin that binds sialic acid on host cell membrane. Neuraminidase is a sialic acid hydrolase that specifically clips off terminally located sialic acid on host cell surface. The two proteins are essential for infectious cycle of influenza virus. During initial infection, an influenza virus will hold onto an epithelial cell through HA-sialic acid interaction. At the end of an infectious cycle, the neuraminidase will cleave the sialic acid on the host cell membrane, releasing the newly formed viral particle from the HA-sialic acid bondage. The neuraminidase activity is also thought to help the virus penetrate mucus. Nine subtypes of NA have been identified, all of which are tetrameric and share a common structure consisting of a globular head, a thin stalk region, and a small hydrophobic region that anchors the protein in the virus membrane.