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). This evidence concerns the gene FGF23 and X-linked dominant hypophosphatemic rickets.