ESR1 and breast cancer: Taking into account the expression of specific molecular biomarkers, such as estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2), breast cancer can be divided into four subtypes, luminal A, luminal B, human epidermal growth factor receptor 2 positive (HER2+) or triple negative (TNBC) (Table 1) [93,94,95,96,97].