These exciting in vitro results showing HER2-TKI induced senescence in breast cancer cells will require further testing, including but not limited to (i) examining additional markers of senescence (e.g., trimethyl-K4 histone H3), (ii) examining TKI-treated breast cancer xenograft tissue samples and critically in TKI-treated patient samples to thoroughly evaluate senescence induction as a novel mechanism of action and potential mechanism of resistance to HER2 targeted TKIs. This evidence concerns the gene ERBB2 and breast cancer.