Trastuzumab, an anti‐HER2 mAb drug approved for the treatment of patients with HER2‐positive breast cancer and gastric cancer (Bang et al., 2010; Slamon et al., 2001), has a direct inhibitory effect on HER2‐positive cancer cells (Barok et al., 2011; Köninki et al., 2010). The gene discussed is ERBB2; the disease is gastric cancer.