Based on molecular characteristics and immunohistochemical (IHC) expression of hormone receptors (HR), BC is commonly grouped into four subtypes: luminal A (ER+, PR+, Ki67 < 20%), luminal B (ER+, PR+ or PR- HER2+ or HER2-, Ki67 > 20%), human epidermal growth factor receptor 2 (HER2)-overexpression (ER+, PR+), and triple-negative BC (TNBC)/basal-like. Here, ESR1 is linked to breast cancer.