Osteonectin, also known as SPOCK1, is an extracellular heparan/chondroitin sulfate proteoglycan. Members of this family are known as testicans, also called SPOCKs. They are characterized structurally by an N-terminal testican-specific domain, a follistatin-like region, a calcium-binding domain, a thyroglobulin-like domain, and an acidic C-terminal domain with two putative glycosaminoglycan attachment sites. SPOCKs are enriched in brain and have been shown to regulate neuronal attachment and outgrowth. They contain inhibitory regions in several domains targeted to different classes of protease, and in some cases may act as protease inhibitors. Osteonectin contains 1 Kazal-like domain and 1 thyroglobulin type-1 domain. Up to now, little is known about osteonectin’s function. It may play a role in cell-cell and cell-matrix interactions. Osteonectin also may contribute to various neuronal mechanisms in the central nervous system.