ESR1 and breast cancer: A new definition of BC molecular subtypes based on immunohistochemical staining was issued via St Gallen International BC Conference 2013: human epidermal growth factor receptor-2 (HER2), Ki67+ < 20%), luminal A (estrogen receptor (ER)+/progesterone (PR)+, luminal B (ER+/PR+ < 20%, HER2, Ki67+ ≥ 20%), HER2 overexpression (ER−, PR−, HER2+), HER2+ B2 (ER/PR+, HER2+), and basal-like TNBC (ER−, PR−, HER2−) [4].