It is necessary to mention that the assessment of TmP/GFR does not allow for discrimination between calcipenic or phosphopenic rickets, as in both entities renal phosphate wasting, and thus reduced TmP/GFR is present, either due to increased PTH (calcipenic rickets), or FGF23 activity or inherited/ acquired impairment of tubular phosphate transporters (phosphopenic rickets) (Table 1). The gene discussed is FGF23; the disease is rickets.