OPN belongs to a family of secreted acidic proteins whose members have an abundance of negatively charged amino acids such as Asp and Glu. OPN also has a large number of consensus sequence sites for post-translational phosphorylation of Ser residues to form phosphoserine, providing additional negative charge. Contiguous stretches of high negative charge in OPN have been identified and named the polyAsp motif (poly-aspartic acid) and the ASARM motif (acidic serine- and aspartate-rich motif), with the latter sequence having multiple phosphorylation sites. This overall negative charge of OPN, along with its specific acidic motifs and the fact that OPN is an intrinsically disordered protein allowing for open and flexible structures, permit OPN to bind strongly to calcium atoms available at crystal surfaces in various biominerals. Such binding of OPN to various types of calcium-based biominerals ‒ such as calcium-phosphate mineral in bones and teeth, calcium-carbonate mineral in inner ear otoconia and avian eggshells, and calcium-oxalate mineral in kidney stones – acts as a mineralization inhibitor by stabilizing transient mineral precursor phases and by binding directly to crystal surfaces, all of which regulate crystal growth.