Bone sialoprotein 2(IBSP) is a monomeric non-collagenous member of the SIBLING family of extracellular matrix proteins. It is principally associated with the early stages of bone mineralization. Mouse IBSP is synthesized as a 324 amino acid (aa) precursor that contains a 16 aa signal sequence and a 308 aa mature region. The mature segment is divided into a basic N-terminus (aa 17 - 62), a central region (aa 63 - 233), and an acidic C-terminus (aa 234 - 317). IBSP is highly glycosylated, sulfated and phosphorylated. Phosphorylation promotes HAp nucleation, while carbohydrate may regulate cell adhesion. IBSP binds tightly to hydroxyapatite, appears to form an integral part of the mineralized matrix. It is probably important to cell-matrix interaction and promotes Arg-Gly-Asp-dependent cell attachment.