ESR1 and invasive breast carcinoma: According to the St Gallen 2013 guidelines43, invasive breast cancer can be classified into five distinct molecular subtypes: luminal A-like (ER+, HER2−, Ki-67 low and PR high), luminal B-like HER2-negative (ER+, HER2−, and either Ki-67 high or PR low), luminal B-like HER2-positive (ER+, PR−, HER2+), HER2-positive (non-luminal) (ER−, PR−, HER2+), and triple-negative (ER−, PR−, HER2−).