This rise in serum FGF-23 concentration appears to be a reflection of an increased production by osteocytes rather than accumulation because of impaired renal function, and it precedes any detectable increase in serum phosphate concentration and the decline in serum 1,25(OH)2D3 levels.(24,55,56) In fact, increased FGF-23 levels in these early CKD stages are associated with increased fractional excretion of phosphate, making it likely that enhanced FGF-23 secretion helps to maintain normophosphatemia despite a reduction in nephron mass (Fig. 1, middle panel). The gene discussed is FGF23; the disease is chronic kidney disease.