The protein encoded by the Bone Sialoprotein gene is a major structural protein of the bone matrix. It is synthesized by skeletal-associated cell types, including hypertrophic chondrocytes, osteoblasts, osteocytes, and osteoclasts, and it constitutes approximately 12 % of the noncollagenous proteins in human bone. The only extraskeletal site of its synthesis is the trophoblast. This protein binds to calcium and hydroxyapatite via its acidic amino acid clusters, and mediates cell attachment through an RGD sequence that recognizes the vitronectin receptor.Synonyms: BNSP, BSP II, Bone sialoprotein II, Cell-binding sialoprotein, IBSP, Integrin-binding sialoprotein