The zona pellucida is an extracellular matrix that surrounds the oocyte and early embryo. It is composed of three glycoproteins with various functions during fertilization and preimplantation development. The glycosylated mature peptide is one of the structural components of the zona pellucida and functions in secondary binding and penetration of acrosome-reacted spermatozoa. Female mice lacking ZP2 (zona pellucida glycoprotein 2) do not form a stable zona matrix and are sterile. Alternative splicing results in multiple transcript variants.