BC can be classified into three main groups based on their molecular and histological differences: BC expressing hormone receptors [estrogen receptor (ER+) or progesterone receptor (PR+)], BC expressing human epidermal receptor 2 (HER2+), and triple negative breast cancer (TNBC) (ER−, PR−, HER2−) [190,191]. Here, ESR1 is linked to triple-negative breast carcinoma.