ESR1 and breast cancer: Breast cancers can be classified based on their gene expression into five subtypes, namely, luminal-A (estrogen receptor (ER)+, progesterone receptor (PR)+, Ki-67low, HER2−), luminal-B (ER+, PR+, Ki-67high, HER2+/−), HER2-positive (ER−, PR−, HER2+), basal (ER−, PR−, HER2−, cytokeratin5/6+, EGFR+), and claudin-low (ER−, PR−, HER2−, claudinlow) (Curtis et al, 2012; Gao et al, 2013; Pereira et al, 2016).