ESR1 and breast cancer: Breast cancer is a heterogeneous disease classified into four subtypes by gene expression profiling, including luminal A (ER/PR+, HER2−, Ki67+ < 20%), luminal B (ER/PR+ < 20%, HER2−, Ki67+ ≥ 20%), HER2 (ER/PR−, HER2 overexpression), and basal-like (ER−, PR−, HER2−) [3,4].