XLH is caused by a loss-of-function mutation in thePHEX(phosphate regulating endopeptidase homolog, X-linked) gene, leading to increased circulating levels of fibroblast growth factor 23 (FGF23) (3,4) and consequent phosphaturia, hypophosphatemia, and low 1,25-dihydroxyvitamin D levels (5,6). This evidence concerns the gene FGF23 and X-linked hypophosphatemia.