ESR1 and breast cancer: We screened a panel of BC derived cell lines for HER2 expression levels by western blot analysis: SKBR3 and BT474 cells, representative of the HER2 + and HER2 + and estrogen receptor+ (ER) subtypes; MCF7, MDA-MB-468, MDA-MB-231 cell lines, representative of the estrogen receptor positive (ER+) and of the triple negative subtypes, respectively; MCF10A cell line from non-tumorigenic breast epithelium was used for comparison (Fig. 1, A).