ESR1 and breast carcinoma: Many of these clinical trials involve the triple-negative breast cancer (TNBC) type, a very aggressive malignant type of tumor that is frequently resistant to standard breast cancer therapies due to the absence of estrogen receptors (ER), progesterone receptors, and epidermal growth factor receptors (EGFR), making novel therapeutic modalities such as CAR T-cells particularly attractive 14.