In the METTEN study assessing the efficacy of adding metformin to neoadjuvant chemotherapy plus trastuzumab in early HER2-positive BC, however, metformin addition failed to show statistically significant superiority in the rate of pCR (66% vs. 59%, p = 0.589) and breast-conserving surgery (79% vs. 59%, p = 0.089) 29. This evidence concerns the gene ERBB2 and breast cancer.