In 2010, the phase 3 ToGA trial first demonstrated the benefit of adding trastuzumab to first-line chemotherapy in patients with HER2-positive (IHC 3+ or FISH amplified HER2/CEP17 ≥ 2) locally advanced, recurrent, or metastatic gastric or GEJ adenocarcinoma [17]. This evidence concerns the gene ERBB2 and gastroesophageal junction adenocarcinoma.