Loss-of-function mutations of the PHEX gene lead to the increase of circulating fibroblast growth factor 23 (FGF23), resulting in hypophosphatemia by reducing resorption of phosphate in the renal proximal tubule and synthesis of 1,25-dihydroxyvitamin D (1,25-(OH)2D3) (4). The gene discussed is FGF23; the disease is hypophosphatemia.