ESR1 and triple-negative breast carcinoma: The MDA-MB-231 is a triple negative breast cancer (TNBC) cell line, associated with poor prognosis from being metastatic, aggressive, and with fewer treatment options available.55 The SK-BR-3 is HER2-positive (estrogen receptor (ER)- and progesterone receptor (PR)-negative) and is associated with the development of resistance against targeted therapies.