ERBB2 and breast cancer: To investigate the effect of metformin on the different subtypes of breast cancer, we investigated its effect on the proliferation and apoptosis of BT-474 (Luminal B), MCF-7(Luminal A), MDA-MB-231(Claudin low), MDA-MB-468 (Basal-like) and SkBr3 (HER2) cell lines.