EPO and renal fibrosis: In adults, kidney‐derived Epo accounts for 90% of the total Epo level in the body.[111] Recent studies have demonstrated that the secretion of Epo by renal EPO‐producing (REP) cells contributes to the progression of renal fibrosis.[62] REP cells constitute a unique subpopulation of tubulointerstitial cells located in the poor O2‐saturated corticomedullary border,[112] with characteristics of fibroblasts, pericytes, and neurons.