These subtypes are grouped into four main categories based on both molecular and histological evidences: the BC-expressing hormone receptor (estrogen receptor (ER+) or progesterone receptor (PR+), which are classified as luminals A and B, respectively), BC-expressing human epidermal receptor 2, classified as HER2+ tumors, and triple-negative breast cancer (TNBC) (ER−, PR−, HER2−) [3,4]. This evidence concerns the gene ESR1 and breast cancer.