ESR1 and breast cancer: Estrogen receptor alpha especially is an important target for drug development for the prevention and treatment of breast cancer [4] and estrogen receptor positive (ER+) tumors show relatively better prognoses for treating breast cancer as compared to ER-negative breast cancer, especially the triple negative breast cancer, which does not express the hormone epidermal growth factor receptor 2 (HER-2), estrogen receptors (ER), and progesterone receptors (PR) [5].