Infection with strains of Helicobacter pylori that carry the cytotoxin-associated antigen A (cagA) gene is associated with gastric carcinoma. Recent studies have shed light on the mechanism through which the cagA gene product, CagA, elicits pathophysiological actions. CagA is delivered into gastric epithelial cells by the bacterial type IV secretion system, where it deregulates the SHP2 oncoprotein. Intriguingly, CagA is noted for its variation, particularly at the SHP2-binding site, which could affect the potential of different strains of H. pylori to promote gastric carcinogenesis.