Aberrant DNA methylation is deeply involved in human cancers, especially those associated with chronic inflammation, such as gastric and colitic cancers. In patients with these cancers, not only cancer tissues but also non-cancerous tissues have high levels of aberrant DNA methylation [Maekita, Clin Cancer Res, 12:989, 2006]. Especially in gastric cancers, accumulation levels have been shown to be correlated with risk of gastric cancers [Nakajima, CEBP, 15:2317, 2006], demonstrating the presence of an epigenetic field for cancerization (epigenetic field defect). The high levels are associated with the presence of H. pylori infection in humans, and infection of Mongolian gerbils demonstrated the causal role of H. pylori infection in methylation induction [Niwa, Cancer Res, 70:1430, 2010]. Inflammation triggered by H. pylori infection, not H. pylori itself, was critical for methylation induction. Comparison among different kinds of inflammation induced by H. pylori infection, ethanol, or salt showed that only inflammation triggered by H. pylori infection was capable of inducing aberrant DNA methylation [Hur, Carcinogenesis, 32:35, 2011]. Although expression of Dnmts was not altered in gastric mucosae exposed to H. pylori infection, expression of Tets was decreased to the half of uninfected gastric mucosae. One important translational aspect is the use of the degree of epigenetic field defect as a cancer risk marker. We isolated seven genes that had large areas under the receiver-operating characteristic curves (0.78-0.84) and high odds ratios (12.7-36.0) compared with two previously isolated markers (0.60-0.65, 5.0-5.7) in a validation cohort (21 gastric cancer patients and 14 healthy individuals) [Nanjo, Gastric Cancer, 15:382, 2012]. Another aspect is to reduce the field defect for cancer prevention. To this end, we treated H. pylori-infected Mongolian gerbils with 5-aza-2'-deoxycitidine (5-aza-dC) (0.125 mg/kg) for 50-55 weeks. The incidence of gastric cancers was decreased from 55.2% (control) to 23.3% (5-aza-dC-treated) (P<0.05). No obvious adverse effect of 5-aza-dC treatment was observed, besides testicular atrophy [Niwa, Cancer Prev Res, 6:263, 2013]. Although removal of aberrant DNA methylation once induced can be associated with demethylation of physiologically methylated sequences, suppression of methylation induction was considered to be a promising method of cancer prevention. The epigenetic field defect, present in tissues exposed to chronic inflammation, provides a rich source of translations.