ESR1 and breast cancer: Breast cancers are classified into four subtypes, i.e., luminal A, luminal B, HER2/ErbB2+ (human epidermal growth factor receptor 2/the erythroblastosis oncogene-B2 positive), and TNBC (triple negative breast cancer), based on estrogen receptor (ER), progesterone receptor (PR), and HER2 expression [23].