ESR1 and breast cancer: Tamoxifen and Aromatase Inhibitors (AIs) currently hold an important place as endocrine therapy (ET) in adjuvant and/or metastatic setting of the patients with Hormone Receptor (i.e. Estrogen Receptor+ and/or Progesterone Receptor+) positive invasive or non-invasive breast cancer (BC) (Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) 2005; Howell et al. 2005).