Thus, hypoxia-induced expansion of ALDH+ cells was observed both in ER-positive and ER-negative cells [27, 28], and increases in the percentage of CD44+CD24− cells and mammosphere formation capacity were shown in ER-negative cells [27, 41, 42], in agreement with our results and in contrast to one report, which detected a decrease in ALDH+ cells and mammosphere formation in ER-negative cancers [30]. Here, ESR1 is linked to cancer.