ESR1 and breast cancer: Two different breast cancer cell lines were used for the experiments; MDA-MB-231 cells derived from triple negative (i.e., no expression of estrogen receptor, progesterone receptor, or HER2) breast cancer, generally classified as more aggressive, treatment resistant cancer, and T-47D cells derived from ductal carcinoma of the breast with estrogen receptor, progesterone receptor, and HER2 expression.