ESR1 and breast cancer: MDA-MB-231 and SUM-149 are triple-negative breast cancer (TNBC) cells that is, they lack the estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor receptor 2 (HER2) and represent two distinct breast cancer subtypes: mesenchymal-like breast cancer (Yao et al., 2011) and epithelial-like inflammatory breast cancer (Zhang et al., 2009), respectively.