 XLH is associated with a mutation in the "PHEX" gene sequence, located on the human X chromosome at location Xp22.2-p22.1.The PHEX protein regulates another protein called fibroblast growth factor 23 (produced from the FGF23 gene).Fibroblast growth factor 23 normally inhibits the kidneys' ability to reabsorb phosphate into the bloodstream.