Runx3 is expressed by gastric epithelial cells throughout development. Mice whose Runx3 gene has been knocked out died soon after birth. In the knockout mouse, gastric epithelia exhibited hyperplasia and epithelial apoptosis was suppressed. Analysis using a primary culture system for the epithelial cells suggested that this is caused by the reduced sensitivity of Runx3−/− gastric epithelial cells to the growth-inhibiting and apoptosis-inducing activities of TGF-β. In human and mouse gastric cancer cell lines, RUNX3/Runx3 was silenced due to hypermethylation of CpG islands in the promoter region. Exogenous expression of RUNX3 in the cells that do not express the endogenous gene caused an inhibition of growth both in vivo and in vitro. These observations indicate that Runx3 is a major growth regulator of gastric epithelial cells, and that it is deeply involved in gastric tumorigenesis in both humans and mice.