ESR1 and breast cancer: Triple negative breast cancer (TNBC) is defined by the absence of estrogen receptor (ER), progesterone receptor (PR) and HER2 amplification and constitutes an exceedingly heterogeneous group of breast cancers, generally stratified into six distinct molecular subtypes including two basal-like subtypes (BL1 and BL2) in addition to immunomodulatory (IM), mesenchymal (M), mesenchymal stem–like (MSL), and luminal androgen receptor (LAR) subtypes [1, 2].