SFRP4 and X-linked hypophosphatemic rickets: However, these regulators alone cannot explain the pathophysiology of X-linked hypophosphatemic rickets (XLH) and other less well-known disorders.92 A group of hypophosphatemic peptides called “phosphatonins” has been identified that include matrix extracellular phosphoglycoprotein (MEPE), secreted frizzled-related protein 4 (sFRP-4), dentin matrix protein 1 (DMP1), fibroblast growth factor-7 (FGF-7), FGF-23, and Klotho.