Alongside numerous studies addressing viral N-glycan composition and function, it was discovered that viral envelope glycoproteins could also be modified with O-linked glycans (Olofsson et al. 1981; Shida and Dales 1981; Niemann et al. 1982; Gruber and Levine 1985; Montalvo et al. 1985; Gong et al. 1987; Lundstrom et al. 1991). As for human proteins, the Golgi apparatus was identified as the site of viral O-glycosylation (Johnson and Spear 1983; Locker et al. 1992). Historically it was presumed that few viruses were O-glycosylated and the function of this modification remained undetermined for some time (Feldmann et al. 1991; Bernstein et al. 1994). One of the first functions of viral O-glycosylation was discovered in vaccinia virus, where it has been demonstrated that the hemagglutinating activity of glycoprotein HA was entirely dependent on O-linked glycans (Shida and Dales 1981). A similar carbohydrate-dependent function was described for rubella virus, where treatment with a mix of glycosidases removing all glycans resulted in inhibition of hemagglutination (Ho-Terry and Cohen 1984).