Trastuzumab-R27T strongly inhibited (Emax = 88.9–100%) HER2-positive cancer cells with high potency (GI50 = 0.006–0.062 nM); however, trastuzumab slightly inhibited (Emax = 14.1–50.4%) HER2-positive cancer cells and had no inhibitory effect on HER2-negative gastric cancer cell lines and the normal gastric epithelial cell line. The gene discussed is ERBB2; the disease is gastric cancer.