X-linked hypophosphatemia (XLH) is a rare, lifelong disease caused by loss-of-function mutations in the PHEX gene, resulting in an excess of FGF23, which impairs renal phosphate reabsorption and suppresses the production of 1,25-dihydroxyvitamin D. This process results in chronic hypophosphatemia and persistent osteomalacia [191]. The gene discussed is PHEX; the disease is X-linked hypophosphatemia.