X-linked hypophosphatemia (XLH: OMIM #307,800) is a genetic disease caused by inactivating mutations in the phosphate regulating endopeptidase homolog, X-linked (PHEX) gene, inducing a fibroblast growth factor-23 (FGF23) mediated hypophosphatemia. The gene discussed is PHEX; the disease is X-linked hypophosphatemia.