ESR1 and breast cancer: Breast cancer is a highly heterogeneous disease, based on the presence of hormone receptor (HR) [estrogen receptor (ER) and progesterone receptor (PR) status], human epidermal growth factor receptor 2 (HER2) status, and the proliferation index Ki67, breast cancer can be classified into four different molecular subtypes [2]: Luminal A, Luminal B, HER2 + , and triple-negative breast cancer (TNBC); according to the intrinsic gene expression characteristics, breast cancer can be classified into five molecular subtypes: Luminal A, Luminal B, Normal-like, HER2-enriched, and Basal-like [3].