ESR1 and breast cancer: Among the different molecular types of breast cancer, triple-negative breast cancer (TNBC) is highly aggressive and has a high incidence of metastasis/relapse and increased mortality due to the absence of the estrogen receptor (ER), progesterone receptor (PR) and human epidermal growth factor receptor 2 (HER2) expression, hindering the ability to employ targeted therapy (Cao et al., 2021).