DPP (also called ?phosphophoryn?) is a highly acidic protein and is the major noncollagenous matrix component of dentin (13,?,15). The molecule is so-called because it is considered to be a ?phosphate carrier? (16). DPP is exceedingly rich in aspartic acid and serine residues ((DSS)n), and about 90% of the serine residues are phosphorylated (17, 18). This enables DPP to have a strong affinity for calcium ion, and thus it significantly promotes the growth of hydroxyapatite crystals when bound to collagen fibrils in vitro.