The role of oncogenes and tumor suppressor genes in the pathogenesis of gastric cancer has recently received considerable attention. p53 is a tumor suppressor gene that is essential in the cell cycle; it prevents G1/S phase transition, after exposure to ionizing radiation or DNA-damaging chemotherapy. This allows the cell to repair its DNA or, if the damage is irreversible, to elicit apoptotic cell death. p53 mutations are seen in many human tumors including gastric carcinoma. Evidence suggests that mutant p53 is associated with shorter life expectancy in gastric, breast, lung, and colorectal cancer. A number of studies have shown cellular resistance to chemotherapy in the presence of mutant p53. Currently, increasing interest has been devoted to the potential role of mutant p53 as a screening tool.