Osteopontin is a secreted molecule that belongs to a family of non-collagenous matricellular proteins. It functions as a ligand to αvβ3 integrin and possibly other receptors. It binds tightly to hydroxyapatite and can act as a structural component of the extracellular mineralized matrix. Osteopontin is initially secreted as a 298 amino acid protein, which is subject to multiple post-translational modifications including glycosylation, phosphorylation, and specific proteolytic cleavages into various smaller molecular weight fragments. Osteopontin is expressed in a wide range of cells and tissues including osteoblasts, various tumor cell lines, extraosseous cells in the inner ear, brain, kidney, deciduum, placenta and odontoblasts.