In HER2-positive breast cancer, a subanalysis of the diabetic population in the ALTTO trial (NCT00490139)—a phase III study that failed to demonstrate the superiority of lapatinib and trastuzumab compared to trastuzumab as adjuvant treatment—identified a positive effect of metformin in terms of disease-free survival, distant disease-free survival, and OS that was limited to the ER-positive population [108]. This evidence concerns the gene ERBB2 and breast carcinoma.