ESR1 and breast cancer: BC, a highly heterogeneous disease, has been clinically divided into three subtypes according to the expression of estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor (HER2), namely, luminal BC (ER+/PR+ and HER2−) and HER2-positive BC (HER2+), as well as triple negative BC (ER−, PR−, and HER2−) [5].