ESR1 and triple-negative breast carcinoma: Among the known four subtypes (luminal-A, luminal-B, HER2-enriched, and triple-negative breast cancer [TNBC]), TNBC and HER2-enriched subtypes display more aggressive behaviors compared with luminal-like disease, which is defined by the moderate to adequate expression of estrogen receptor (ER) and/or progesterone receptor (PR).