ESR1 and breast cancer: To verify that the selected breast cancer cell lines represented the expected subtypes, we performed mRNA-seq (Supplementary Table 1) and quantified the expression levels of the three receptors conventionally used for breast cancer subtyping: estrogen receptor (ESR1, also known as ER), progesterone receptor (PGR, also known as PR), and human epidermal growth factor receptor 2 (ERBB2, also known as HER2)22,23.