ESR1 and breast cancer: Based on the expression of the estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER2), as well as a proliferation marker Ki67, breast cancer can be divided into four intrinsic molecular subtypes: luminal A (ER+, PR+, HER2−, Ki67Low), luminal B (ER+, PR+, HER+/−, Ki67High), HER2-enriched (ER−, PR−, HER2+), and basal-like subtype (ER−, PR−, HER2−), which largely resembles triple-negative breast cancer (TNBC) and comprises approximately 15% of all breast cancer cases [12].