EPO and anemia (phenotype): However, only up to 10% of REP cells are ON-REP cells, even under very severe chronic anemia conditions, suggesting that most REP cells are reservoirs (referred to as OFF-REP cells) in preparation for much more severe conditions that require high amounts of EPO (Figures 1C, D; Yamazaki et al., 2013; Souma et al., 2015).