ERBB2 and breast cancer: It has been proposed that the predominant mechanism of action of metformin may differ across BC molecular subtypes 32, yet at the cellular level, metformin has been shown to inhibit growth in trastuzumab-resistant HER2+ cells 33 and to inhibit cell proliferation and induce apoptosis in triple-negative BC cell lines 34, suggesting that the effect of metformin in the Bayraktar et al.