For patients with gastric cancer showing HER2 overexpression (identified as either a HER2 immunohistochemistry (IHC) score of 3+ or a 2+ IHC score plus a positive fluorescence in situ hybridization (FISH) test), the addition of trastuzumab, an anti-HER2 antibody, to conventional platinum and fluoropyrimidine-based chemotherapy (ChT) has proven beneficial. The gene discussed is ERBB2; the disease is gastric cancer.