MUC1is a highly glycosylated, type I transmembrane protein expressed by normal ductal epithelial cells of the pancreas, breast, lung, and gastrointestinal tract, and overexpressed in many cases of adenocarcinoma.We down-regulated MUC1expression by RNA interference and investigated the effects on malignant and metastatic potential of a human pancreatic cancer cell line, S2-013. MUC1-suppressed clones, S2-013.MTII.C1and S2-013.MTII.C2, were established by targeting a sequence 3,151 bp from the initiation codon and characterized in vitro for proliferation, invasion, and adhesion. We evaluated the effects of MUC1 suppression in vivo on tumor growth and metastatic properties following implantation into the cecum or pancreas of athymic mice. MUC1-suppressed clones showed significantly decreased proliferation in vitro and in vivo. Global gene expression was evaluated by oligonucleotide microarray analysis. Surprisingly, genes predicted to increase doubling times (cyclin B1 and cyclin D3) were overexpressed in MUC1-suppressed clones. There were alterations in expression of several genes that may affect the malignant properties of pancreatic cancer. Adhesion of MUC1-suppressed cells in vitro to type IV collagen and fibronectin was slightly increased, and adhesion was slightly decreased to type I collagen and laminin. Results of implantation to cecum and pancreas showed significant reduction of metastasis to lymph nodes, lung, or peritoneal sites compared with S2-013.gfp-neo control cells. These results support the hypothesis that MUC1 contributes significantly to growth and metastasis, and that down-regulation of MUC1 protein expression decreases the metastatic potential of pancreatic adenocarcinoma.In spite of recent efforts to improve prevention, screening, and therapy, pancreatic cancer has a poor prognosis: a 5-year survival rate of f3% and a median survival of <6 months (1). The poor prognosis is a consequence of metastatic disease that results from a lack of early detection and effective treatment. MUC1 is a polymorphic, highly glycosylated, type I transmembrane protein expressed by ductal epithelial cells of secretory organs, including the pancreas, breast, lung, and gastrointestinal tract (2), which is overexpressed and aberrantly glycosylated in most cases of adenocarcinoma. Previously, we investigated the role of MUC1 in invasion and metastasis by heterotopic implantation of tumor fragments onto the cecum of nude mice (3). Overexpressing full-length MUC1 in the pancreatic tumor cell line, S2-013, which expresses low levels of endogenous MUC1 and is spontaneously metastatic in the nude mouse model, altered the propensity for these cells to metastasize to lymph nodes and lungs.Overexpressing two mutant forms of MUC1, tandem repeat deleted [MUC1(CTR)] or cytoplasmic tail deleted (MUC1F.CT3), eliminated the phenotype and restored to control levels the degree of metastasis to lymph nodes and lung. DNA microarray analyses on clonal populations of these cells revealed very few differences in gene expression p...