ESR1 and breast carcinoma: Further apoptosis analysis using another HER2 breast cancer cell line MDA-MB-453 resulted in similar results to the ones obtained with SkBr3 (Additional file 1: Figure S1A) These results demonstrate that the SkBr3 breast cancer cells, characterised by the HER2+/ER−/PR- phenotype (HER2), are more resistant to metformin as they were less affected by the apoptotic effect of metformin and could maintain a higher proliferation ability compared to the other breast cancer cell lines.