ESR1 and breast cancer: In contrast, MDA-MB-231 isinvasive, poorly differentiated and highly aggressive triple-negativebreast cancer cell line without HER2 amplification as well as estrogenreceptor (ER) and progesterone receptor (PR) expression.26,27 Triple-negative breast cancers are usually more aggressive, moredifficult to treat, and more likely to recur than cancers that arehormone-receptor-positive or HER2-positive.