This family represents a series of 13 reside repeats found in the apopolysialoglycoprotein of Oncorhynchus mykiss (Rainbow trout) and Oncorhynchus masou (Cherry salmon). Polysialoglycoprotein (PSGP) of unfertilised eggs of rainbow trout consists of tandem repeats of a glycotridecapeptide, Asp-Asp-Ala-Thr*-Ser*-Glu-Ala-Ala-Thr*-Gly-Pro-Ser- Gly (* denotes the attachment site of a polysialoglycan chain). In response to egg activation, PSGP is discharged by exocytosis into the space between the vitelline envelope and the plasma membrane, i.e. the perivitelline space, where the 200-kDa PSGP molecules undergo rapid and dramatic depolymerisation by proteolysis into glycotridecapeptides [1].

1: Molecular cloning and characterization of cDNAs coding for apo-polysialoglycoprotein of rainbow trout eggs. Multiple mRNA species transcribed from multiple genes contain diverged numbers of exact 39-base (13-amino acid) repeats. Sorimachi H, Emori Y, Kawasaki H, Kitajima K, Inoue S, Suzuki K, Inoue Y; J Biol Chem 1988;263:17678-17684. PMID:3182867