ERBB2 and breast cancer: Breast cancer is categorised into four molecular subtypes based on immunohistochemistry (IHC): i) luminal A (ER+, PR+, HER2-, Ki67 < 20%); ii) luminal B (ER+, PR+ and HER2+ and Ki67≥ 20%); iii) HER2-enriched (ER−, PR−, HER2+); and iv) basal-like (triple-negative: ER−, PR−, HER2) (4).