ESR1 and breast cancer: It can be classified by molecular and histological characteristics into three groups: BC expressing hormone receptor (estrogen receptor (ER+) or progesterone receptor (PR+), BC expressing human epidermal receptor 2 (HER2+) and triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) [11], with TNBC being the most invasive [12].