Zolbetuximab, a chimeric (mouse/human IgG1) monoclonal antibody targeting claudin-18 isoform 2 (CLDN18.2), is a novel therapeutic agent for CLDN18.2-positive, human epidermal growth factor receptor 2 (HER2)-negative locally advanced unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma. This evidence concerns the gene ERBB2 and gastroesophageal junction adenocarcinoma.