ESR1 and breast cancer: Breast cancer is a heterogeneous disease that can be divided into clinically defined subtypes including estrogen receptor (ER)-positive, HER2-positive, and triple-negative breast cancer (TNBC, which lack ER, progesterone receptor (PR) and HER2), or molecularly defined subtypes including luminal A, luminal B, HER2-enriched and basal-like breast cancer (BLBC).