Circulating FGF-23 levels are already elevated in patients with CKD stages 2 and 3, that is, well before there is a critical reduction in functioning nephrons [CKD stages 1 and 2: glomerular filtration rate (GFR) > 60 mL/min per 1.73 m2; stage 3: 30 to 59 mL/min per 1.73m2, stage 4: 15 to 29 mL/min per 1.73m2; stage 5: <15 mL/min per 1.73m2]. The gene discussed is FGF23; the disease is chronic kidney disease.