Inhibition of HIF-1α resensitised the ERα positive breast cancer cell lines to tamoxifen (Nadai et al., 2021) Exploring cancer stem cell (CSC) activity using in vitro and in vivo models, including patient-derived tumour samples, Harrison et al. demonstrated HIF-1α-dependent CSC activity in hypoxic ERα positive cancers, a phenomena that was inhibited by blocking oestrogen and Notch signalling. Here, ESR1 is linked to cancer.