The molecular classification of BC based on gene expression divides BC into five types: (1) luminal A (HR+, HER2-, and lower proliferation index), comprising 30%-40% of all BC, (2) luminal B (low ER+, PR-, HER2+, and high proliferation index), comprising 30%-40% of all BC, (3) HER2+ and HR-, comprising 15%-25% of all BC, (4) basal-like triple-negative, comprising 10%-20% of all BC, and (5) Claudin-low, which is usually triple-negative and comprises 10%-15% of all BC. Here, ESR1 is linked to breast cancer.