Here we explored the impact of metformin on serum metabolic profiles of patients participating in the METTEN study, a phase 2 clinical trial of HER2-positive breast cancer patients randomized to receive either metformin combined with anthracycline/taxane-based chemotherapy and trastuzumab or an equivalent regimen without metformin, before surgery [26]. This evidence concerns the gene ERBB2 and breast carcinoma.