ESR1 and breast cancer: Triple-negative breast cancer (TNBC), which accounts for 20% of all BC, is a type of BC that is negative for estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER-2), resulting in a meager survival rate due to its aggressiveness, malignancy and poor prognosis (Oh et al., 2017; Hon et al., 2016).