PGR and breast cancer: Cell lines were chosen to reflect four major breast cancer subtypes [15, 16]: MCF7 (ER+/HER2−, luminal A), ZR-75-1 (ER+/HER2+, luminal B), SKBR3 (ER−/HER2+, HER2-amplified) and MDA-MB-231 (ER−/progesterone receptor–negative [PR−]/HER2−, triple-negative), and they were exposed to increasing concentrations of epirubicin until resistant cells were generated.