ESR1 and breast carcinoma: To better replicate breast cancer heterogeneity, three different breast cell lines were used: MDA-MB-231, a highly invasive triple-negative breast cancer cell line, i.e., cells are negative for hormone receptors (estrogen and progesterone) and HER2; MCF-7, a less invasive luminal cell line that is positive for estrogen receptors, but negative for HER2 (Dai et al., 2017); and MCF10A, a non-tumorigenic breast cell line.