Zona pellucida glycoprotein mZP3 is a structural glycoprotein, a sperm receptor, and an acrosome reaction-inducer. The latter two functions are mediated, at least in part, by mZP3 oligosaccharides. mZP3 is unique to mammalian eggs, from mice to humans, although related glycoproteins are found in vitelline envelopes of a variety of eggs, from fish to birds.