X-linked hypophosphatemia (XLH) (ORPHA:89936) is a rare, heritable, progressive, and lifelong disorder caused by increased circulating levels of fibroblast growth factor 23 (FGF23), a phosphate-regulating hormone that leads to reduced renal phosphate reabsorption and consequent abnormal bone mineralization. The gene discussed is FGF23; the disease is X-linked hypophosphatemia.