ESR1 and neoplasm: Related to resistance due to the tumor subtype, Davis and colleagues71 reported that the MCF7 cell line (ERα+/PR+/HER2−), representative of the luminal A tumor subtype, is sensitive to DOX, while the MDA-MB231 cell line (ERα−/PR−/HER2−), representative of the triple negative tumor subtype, is resistant to DOX.