ESR1 and breast cancer: Breast cancer is a heterogeneous disease classified into four subtypes by protein expression in tissues, including luminal A (estrogen receptor (ER) or progesterone receptor (PR)+, human epidermal growth factor receptor 2 (HER-2)−, or Ki67+ < 20%), luminal B (ER or PR+, HER-2+, or Ki67+ ≥ 20%), HER-2 (ER or PR−, HER-2 +), and basal-like (ER−, PR−, HER-2−) [2].