EPO and cancer: Although liver‐ and brain‐derived cancer cell lines with permanent hypoxia‐inducible Epo expression are well‐established (Orlando et al., 2020), according to our and others' experience primary REP cells and nonconditionally immortalized REPD cell lines tend to lose hypoxia‐inducible Epo production during in vitro cultivation (Bussolati et al., 2013; Imeri et al., 2019; Maxwell et al., 1993; Olmos et al., 2018; Sato, Hirano, et al., 2019), consistent with the “on‐off” behavior of REP cells in vivo (Souma et al., 2013, 2015; Suzuki & Yamamoto, 2016).