Human Osteopontin （OPN） is a negatively charged hydrophilic protein of 314 amino acids and is subject to significant post translational modifications （PTM） including phosphorylation and glycosylation. Due to its acidic nature and PTMs，OPN runs anomalously by SDS-PAGE. Although its mass is 35kD，apparent molecular weights may range up to 75kD. In addition，OPN is subject to proteolytic modification into smaller molecular weight fragments. The matricellular protein osteopontin binds to cell surface receptors and is secreted into many body fluids including milk，blood and urine，depending on the organ of origin. This makes osteopontin an ideal candidate for being a biomarker as the secreted form is easily obtained in throwaway fluids，and mimics the cellular environment from which it is released. Osteopontin is important in immune responses and inflammation as well as bone generation and remodeling. In autistic children，serum levels of osteopontin are correlated to the severity of disease，probably due to a brain inflammation pattern in these children. In aortic valve sclerosis and stenosis，increased levels of secreted osteopontin are also noted. Osteopontin has also been suggested as a cancer biomarker，since it is associated with tumor formation，progression and metastasis. In bone and tooth formation osteopontin is known to be a negative regulator of parathyroid hormone-related protein receptor，which induces osteogenesis. Without appropriate levels of osteopontin，bone growth continues unregulated，and leads to specific bone cancers. In short，osteopontin is a strong marker for bone growth，inflammation and certain cancers. The newly exposed SVVYG epitope on the N-terminal fragment has also been shown to participate in cell adhesion.
