ESR1 and breast cancer: According to immunohistochemistry (IHC) assessment of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor-2 (HER2), together with clinicopathological features, breast cancer can be divided into the following subtypes: luminal A (ER+ and/or PR+, HER2−, low Ki67), luminal B (ER+ and/or PR+, high Ki67 or HER2+), HER2-amplified (HER2+, ER− and PR−), and triple-negative breast cancer (TNBC) (ER−, PR−, and HER2−) [2].