ESR1 and breast cancer: We selected four breast cancer cell lines representing all combinations of ERα and HER2 expression: the luminal-like cells BT474ER+HER2+, SKBR3ER-HER2+ and MCF-7 ER+HER2-, which represent the majority of breast cancers and the basal-like cell line MDA-MB-231ER-HER2- [46].