Using the differential display technique, we have recently reported the identification of a novel gene originally designated DD96. As determined by Northern blot and in situ hybridization, DD96 was expressed at significant levels only in a single epithelial cell population, the proximal tubular epithelial cells of the kidney. However, it was diffusely expressed in various carcinomas originating from kidney, colon, lung, and breast. Using a specific polyclonal antibody, we have not determined that the DD96 protein product is a 17-kd membrane-associated protein, which we have therefore redesignated MAP17. In normal tissues, MAP17 is expressed in significant amounts only in the kidney, where it was localized to the brush border of proximal tubular epithelial cells. However, MAP17 is expressed abundantly in carcinomas arising from kidney, colon, lung, and breast, in some cases with a membrane-associated apical glandular distribution. In tissue culture, MAP17 was localized to the cell membrane in areas of cell-cell contact, ie, the distribution of cell-function-associated proteins. Transfection of a full-length wild-type DD96 cDNA clone into a colon carcinoma cell line, HT-29, markedly decreased cell proliferation in vitro and tumor growth in vivo. Although the precise function of MAP17 remains to be determined, our findings suggest that this protein may play an important role in tumor biology.