XLH is caused by a loss-of-function mutation in the phosphate-regulating endopeptidase gene, X-linked (PHEX), and results in overactivity of fibroblast growth factor 23 (FGF23).82 The excess circulating FGF23 inhibits vitamin D 1α-hydroxylation and phosphate reabsorption by the kidneys, leading to hypophosphatemia and defective mineralization of the bones and thus facilitating rickets and osteomalacia.83 The gene discussed is FGF23; the disease is hypophosphatemia.