XLHR is caused by mutations in the phosphate-regulating gene with homologies to endopeptidases on the X chromosome (PHEX; OMIM #300550) (3), which is characterized by hypophosphatemia, aberrant vitamin D metabolism, growth retardation, and bone mineralization defects and is associated with a pathological elevation of FGF23 (4). The gene discussed is FGF23; the disease is hypophosphatemia.