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BACKGROUND & AIMS:Rectosigmoid villous adenomas (RVAs) account for approximately 10% of all colorectal tumors. They have distinct pathological features, including abundant mucus secretion, possible malignant transformation, and multiple recurrences after conservative treatment. The aim of this study was to determine the nature of any changes in mucin gene expression in RVAs. METHODS:In situ hybridization was used to examine mucin messenger RNA expression in a series of 22 patients with an RVA. Five normal rectal and colonic mucosae and five rectal adenocarcinomas were used as controls. RESULTS:In the 22 RVAs, we found an overexpression of MUC2 and an aberrant expression of MUC5AC. This MUC5AC expression was more intense in RVAs with low-grade dysplasia than in those cases with high-grade dysplasia. Moreover, in 4 cases, it was detected at a distance from the tumor in areas previously considered as normal by endoscopic and histological examination. CONCLUSIONS:MUC5AC seems to be a specific marker for RVAs and thus may be useful for the early detection of RVA recurrences after endoscopic laser treatment. (Gastroenterology 1996 Jan;110(1):84-91)