ESR1 and breast cancer: Breast cancer is categorized into the following subtypes based on the expression level of estrogen receptor (ER), progesterone receptor (PR), and human epidermal receptor 2 (HER2): basal‐like breast cancer, luminal A, luminal B, triple-negative breast cancer (TNBC), and human epidermal growth factor receptor 2 (HER-2) amplified subtype (2).