Firstly, using MDA-MB-468 cells, we observed a significant increase in CD44+CD24−/lowESA+ cells, which reached a plateau by 48-72 hours treatment (Supplementary Figure 2A) and, therefore, we evaluated the effect of 3-day long hypoxia treatment on the CSC populations in a panel of ER-positive and ER-negative breast cancer cell lines. Here, CD44 is linked to breast cancer.