Specifically, taking advantage of 27 well-studied breast cell lines (11 ER positive and 16 ER negative) whose gene expressions are publicly available, we tested their sensitivity to 7 chemotherapy agents commonly used singly or in combination to treat breast cancer patients: cyclophosphamide, docetaxel, doxorubicin, epirubicin, fluorouracil, gemcitabine, and paclitaxel. This evidence concerns the gene ESR1 and breast cancer.