ESR1 and breast cancer: Gallen International Breast Cancer Conference, breast cancer was classified according to molecular subtype: luminal A [estrogen receptor (ER)+ and/or progesterone receptor (PR)+, Ki67 low, and human epidermal growth factor receptor 2 (HER2)-]; luminal B (ER+ and/or PR+, Ki-67 high and/or HER2+); HER2-positive (ER-, PR- and HER2+); triple-negative types (ER-, PR-, and HER2-) [2].