This report is consistent with our results, as we observed that the MCF7 and ZR751 cell lines, both ERα+/PR+/HER2− and representative of the luminal A BC subtype, are sensitive to DOX treatment, while the MDA-MB468 (ERα−/PR−/HER2−) and KPL4 cell lines (ERα−/PR−/HER2+), representatives of triple negative and HER2+ BC subtype, respectively, did not respond to this drug. This evidence concerns the gene ESR1 and breast cancer.