ESR1 and breast cancer: Considering the fact that the endocrine therapy is effective in the treatment of ER + breast cancer patients, resistance develops in around 25% of early-stage patients and virtually all metastatic patients, results in a poor clinical prognosis (Liu et al., 2006; Serrano-Gomez et al., 2016; Bell et al., 2019); (Jansen et al., 2005; Generali et al., 2006; Pontiggia et al., 2009).