ESR1 and breast cancer: Based on gene expression profiling, breast cancer can be categorized into four main molecular sub-types: (i) basal-like breast cancer (mainly estrogen-receptor (ER)-negative, progesterone-receptor (PR)-negative, and human epidermal growth factor receptor 2 (HER2)-negative); (ii) luminal-A cancer (ER-positive or ER+, and histologically low-grade); (iii) luminal-B cancer (ER+ and histologically high-grade); and (iv) HER2-positive (HER2+) cancer (over-expression and/or amplification of HER2).