ESR1 and breast cancer: Based on expression of receptors such as estrogen receptor (ER), progesterone receptor (PR) and HER2, breast cancers are classified clinically into luminal A (ER+PR+HER2−), luminal B (ER+PR+HER2+/− and/or Ki67high), HER2 positive (ER−), and triple negative tumors lacking expression of all three receptors [186].