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AIM: Hypermethylation of the promoter of the hMLH1gene, which plays an important role in mismatch repair during DNA replication, occurs in more than 30% of human gastric cancer tissues. The purpose of this study was to investigate the effects of environmental factors, genetic polymorphisms of major metabolic enzymes, and microsatellite instability on hypermethylation of the promoter of the hMLH1 gene in gastric cancer.METHODS: Data were obtained from a hospital-based,case-control study of gastric cancer. One hundred and ten gastric cancer patients and 220 age- and sex-matched control patients completed a structured questionnaire regarding their exposure to environmental risk factors.Hypermethylation of the hMLH1 gene promoter,polymorphisms of the GSTM1, GSTT1, CYP1A1, CYP2E1,ALDH2 and L-myc genes, microsatellite instability and mutations of p53 and Ki-ras genes were investigated.RESULTS: Both smoking and alcohol consumption were associated with a higher risk of gastric cancer with hypermethylation of the hMLH1 gene promoter. High intake of vegetables and low intake of potato were associated with increased likelihood of gastric cancer with hypermethylation of the hMLH1 gene promoter. Genetic polymorphisms of the GSTM1, GSTT1, CYP1A1, CYP2E1,ALDH2, and L-mycgenes were not significantly associated with the risk of gastric cancer either with or without hypermethylation in the promoter of the hMLH1 gene.Hypermethylation of the hMLH1 promoter was significantly associated with microsatellite instability (MSI): 10 of the 14 (71.4%) MSI-positive tumors showed hypermethylation,whereas 28 of 94 (29.8%) the MSI-negative tumors were hypermethylated at the hMLH1 promoter region.Hypermethylation of the hMLH1 gene promoter was significantly inversely correlated with mutation of the p53gene.CONCLUSION: These results suggest that cigarette smoking and alcohol consumption may influence the development of hMLH1-positive gastric cancer. Most dietary factors and polymorphisms of GSTM1, GSTT1,CYP1A1, CYP2E1, ALDH2, and L-myc genes are not independent risk factors for gastric cancer with hypermethylation of the hMLH1 promoter. These data also suggest that there could be two or more different molecular pathways in the development of gastric cancer, perhaps involving tumor suppression mechanisms or DNA mismatch repair.