ESR1 and triple-negative breast carcinoma: The MDA-MB-231 cell line, a triple-negative breast-cancer cell which does not express estrogen receptor (ER), progesterone receptor (PR), or human epithelial receptor 2 (HER2), is considered to be representative of a highly aggressive type of cancer cell, whereas MCF-7 expresses ER and PR and is considered a less aggressive cancer cell.