OPN is a substrate protein for a number of enzymes whose actions may modulate the mineralization-inhibiting function of OPN. PHEX (phosphate-regulating gene with homologies to endopeptidases on the X chromosome) is one such enzyme, which extensively degrades OPN, and whose inactivating gene mutations (in X-linked hypophosphatemia, XLH) lead to altered processing of OPN such that inhibitory OPN cannot be degraded and accumulates in the bone (and tooth) extracellular matrix, likely contributing locally to the osteomalacia (soft hypomineralized bones) characteristic of XLH.