Using constitutive Epo-Cre REP reporter mice, a large increase of the proportion of tagged REP cells positive for α smooth muscle actin (αSMA) has been observed [100], leading to the proposal that REP-to-myofibroblast transdifferentiation is the main cause of the loss of Epo expression during CKD [99, 110]. The gene discussed is EPO; the disease is chronic kidney disease.