ESR1 and breast cancer: Luminal A breast cancers are positive for the estrogen receptor (ER) and/or for the progesterone receptor (PR) but not for the Human Epidermal Growth Factor Receptor 2 (HER2); luminal B are ER+ and/or PR+ and HER2+/−, with a general higher proliferation rate; HER2 breast cancers are only HER2+, while TNBCs are ER-, PR- and HER2-.