ESR1 and breast cancer: While ER-positive, PR-positive, and HER2-positive breast cancer tend to have more tailored treatments, in the case of ER-negative or triple-negative (ER-negative, PR-negative, and human epidermal growth factor receptor 2 (HER2)-negative) breast cancer (TNBC), there are almost no targeted therapies available so far, and options are limited to routine chemotherapies [58].