Watanabe et al. have demonstrated that, even though trastuzumab inhibits ERK1/2 phosphorylation in SKBR3 HER2-positive BC cells, it does not have any effect on ERK1/2 phosphorylation in BT474 HER2-positive BC cells [58], suggesting the cell line-specific effects of trastuzumab on MAPK signaling. The gene discussed is MAPK3; the disease is breast cancer.