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It has been proposed that the transcription factor RUNX3 is the product of a gastric tumour suppressor gene. We examined RUNX3 expression in gastric biopsies from 105 patients with different histological presentations. Surprisingly, immunohistochemical staining detected RUNX3 protein expression only in infiltrating leukocytes but not in the gastric epithelium. Using laser capture microdissection and quantitative reverse transcription-polymerase chain reaction, we confirmed that the level ofRUNX3mRNA expression in the gastric epithelium was very low and was influenced neither byH. pyloriinfection nor by neoplastic transformation. Instead,RUNX3was highly expressed in the gastric stroma and the level of expression correlated with the magnitude ofH. pylori-induced gastric inflammation. The low level of RUNX3 expression in gastric epithelium and the absence of downregulation in gastric cancer do not support the hypothesis that RUNX3 functions as a gastric tumour suppressor gene. Copyright © 2006 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.