ESR1 and breast cancer: Based on the standard receptor notation for breast cancer (estrogen receptor, ER; progesterone receptor, PR; Her2 receptor), we have used the following cell lines: MCF-7 (ER/PR double positive), MDA-MB-231 (triple negative), BT-20 (Triple negative), AU-565 (Her2 positive), MCF10A (non-malignant breast epithelial cell lines).