ESR1 and breast cancer: Based on genetic profiling, breast cancer is classified into four subtypes: luminal A (ER+ and/or PR+, HER2−, low Ki67 index), luminal B (ER+ and/or PR+, HER2+ or −, high Ki67 index), HER2 positive (HER2+, ER−, PR−), and triple negative (ER−, PR−, and HER2−) [2].