ESR1 and breast cancer: Breast cancer is treated based on subtype defined by hormone receptors and human epidermal growth factor receptor 2 (HER2) receptor expression; pharmacological inhibition of 17β-estradiol (E2) production or binding of E2 to estrogen receptor (ER) is an effective treatment for patients with ER-positive breast cancer, and ER status is an important prognostic factor [207].