ESR1 and breast cancer: In the clinical setting, breast cancer is comprised of three main subtypes: HER2-positive, estrogen receptor (ER)-positive luminal, and basal-like cancers (BLBCs) that make up the majority of triple-negative breast cancer [TNBC; ER, PR, HER2 negative by immunohistochemistry (IHC) analysis] [1, 2].