H. pylori-induced gastritis plays important roles in gastric cacrcinogenesis and cancer development is characterized by accumulation of various gene aberrations. Activation-induced cytidine deaminase (AID) is exclusively expressed in B lymphocytes and is essential for diversification of immunoglobulin genes. H. pylori-positive human gastritis mucosa ectopically express AID. H. pylori infection to gastric cells in vitro induced AID in association with induction of various gene mutations and deletions. H. pylori-induced AID is mediated by NFkappaB. Furthermore, AID gene introduction into gastric mucosal cells accelerated p53 gene mutations, and inhibition of AID using siRNA significantly reduced H. pylori-induced p53 gene mutations. Thus, H. pylori infection induces ectopic AID expression in the gastric mucosal cells via NFkappaB activation, resulting in various gene mutations and aberrations in the gastric mucosal cells leading to gastric cancer development.