ESR1 and breast cancer: BC, especially triple-negative BC, which is defined by no or low expression of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor 2, usually metastasizes from primary tumors to distant sites, such as the brain, lung, liver, and bone, thereby resulting in a poor prognosis and high mortality (Cheung and Ewald, 2014; Couch et al., 2015; Boire et al., 2020).