High FGF23 levels cause urinary phosphate wasting by down-regulating the renal sodium phosphate transporters 2a and 2c (NaPi2a and NaPi2c, respectively) and reducing transformation of 25-OH-Vitamin D3 to the active vitamin D form 1,25-OH2-Vitamin D3 (4, 5, 6) leading to abnormal low levels of 1,25-OH2-Vitamin D3 despite of hypophosphatemia. This evidence concerns the gene FGF23 and hypophosphatemia.