ESR1 and breast cancer: Currently, breast cancer is recognized as a group of highly heterogeneous diseases and is further categorized into three major different subtypes based on immunohistochemical expression of receptors: triple-negative [estrogen receptor (ER) negative, progesterone receptor (PR) negative, and human epidermal growth factor receptor 2 (HER2) negative], HER2-positive (HER2+; ER and PR + or -), and ER-positive (ER+, HER2-, PR + or -) [1,2].