ESR1 and breast cancer: The nonsteroid antiestrogen tamoxifen [trans-1-(4-β-dimethylaminoethoxy-phenyl)-1,2-diphenylbut-1-ene] (Fig. 1) is the most commonly used endocrine treatment for estrogen receptor α (ERα)-positive breast cancer in pre- and post-menopausal women, and it has helped to reduce breast cancer death rate by one third [1]–[3].