ESR1 and breast cancer: Molecular breast cancer subtypes include normal-like (ER+, PGR+, HER2−, Ki67−), basal (ER−, PGR−, HER2−, basal marker+), luminal subtype A (LumA; ER+, PGR+, HER2−, Ki67−), luminal subtype B (LumB; ER+, PGR+, HER2−, Ki67+), HER2-positive (ER−, PGR−, HER2+), and triple-negative (ER−, PGR−, HER2−) [2].