Although Fgf21−/− CKD mice developed severer kidney damage than wild-type CKD mice (Fig. 1d–f), this did not explain the poor prognosis of Fgf21−/− CKD mice, because restoration of their circulating FGF21 levels by administration of an FGF21-expressing adeno-associated virus vector (AAV8-FGF21) improved their survival (Fig. 1k) without improving the kidney damage (Fig. 1d–f). This evidence concerns the gene FGF21 and chronic kidney disease.