This family represents the N-terminal Ig-like domain from a number of zona-pellucida-binding proteins that seem to be restricted to mammals. These are sperm proteins that bind to the 90-kDa family of zona pellucida glycoproteins in a calcium-dependent manner [1]. These represent some of the specific molecules that mediate the first steps of gamete interaction, allowing fertilisation to occur [2].

1: Amino acid sequences of porcine Sp38 and proacrosin required for binding to the zona pellucida. Mori E, Kashiwabara S, Baba T, Inagaki Y, Mori T; Dev Biol 1995;168:575-583. PMID:7729589

2: Molecules involved in mammalian sperm-egg interaction. McLeskey SB, Dowds C, Carballada R, White RR, Saling PM; Int Rev Cytol 1998;177:57-113. PMID:9378618