Numerous reports show elevation of FGF-23 in some – but not all – patients with TIO.2, 3 Removal of the tumor is associated with reduction in serum FGF-23 concentrations, and there is a temporal association between the reduction in FGF-23 concentration and the elevation in serum phosphate, decrease in renal phosphate wasting, and increase in 1α,25(OH)2D3 concentrations.6 Elevated levels of FGF-23 may not be diagnostic, as patients with X-lined hypophosphatemia may also have raised levels of this peptide. The gene discussed is FGF23; the disease is hypophosphatemia.