ESR1 and breast cancer: The luminal subtype was further classified by Sorlie et al. [11] as luminal A and luminal B. Waks et al. [12] classified BC into three major subtypes based on hormone bimolecular markers, oestrogen receptor (ER), progesterone receptor (PR) and HER2 (hormone receptor 2): ER+/PR+/HER2−, HER2+ and TNBC.