ESR1 and breast cancer: Human breast cancer cell lines with varying endogenous expression levels of ER and HER2 (MCF-7 (ER+, HER2−), ZR75.1 (ER+, HER2++), BT474 (ER+, HER2+++), SKBR3 (ER−, HER2+++)) were genetically engineered to express aromatase (A) or the backbone vector (neo) (Banerjee et al, 2009) (Supplementary Figure 1).