XLH is caused by loss of function of the gene PHEX (2), a phosphate regulating gene with homologies to endopeptidases, leading to elevated circulating levels of fibroblast growth factor 23 (FGF23), renal wasting of phosphate and defective renal production of 1,25-dihydroxyvitamin D (3). The gene discussed is PHEX; the disease is X-linked hypophosphatemia.