Neuraminidase enzymes are glycoside hydrolase enzymes (EC3.2.1.18) that cleave the glycosidic linkages of neuraminic acids. Neuraminidase enzymes are a large family, found in a range of organisms. The best-known neuraminidase is the viral neuraminidase, a drug target for the prevention of the spread of influenza infection. The viral neuraminidases are frequently used as antigenic determinants found on the surface of the Influenza virus. Some variants of the influenza neuraminidase confer more virulence to the virus than others. Other homologs are found in mammalian cells, which have a range of functions. At least four mammalian sialidase homologs have been described in the human genome (see NEU1, NEU2, NEU3, NEU4).