ESR1 and breast cancer: Moreover, based on their molecular characteristics, like the expression of progesterone receptor (PR), estrogen receptor (ER), and human epidermal growth factor receptor 2 (HER2), breast cancers are further categorized into luminal A (ER+ PR+, and HER2-), luminal B (ER+/PR+ and HER2+/−), HER2 positive (ER−/PR− and HER2+), triple-negative/basal-like (ER−, PR−, and HER2−), and claudin-low [4,5,6].