Although peritoneal dissemination is a major obstacle in treating diffuse-type GC patients, the mechanism is still unclear. To address this phenomenon from the viewpoint of molecular biology, we established a highly peritoneal-metastatic cell line, 60As6, from a diffuse-type GC derived cell line, HSC-60, through a six-times iterative in vivo selection consisting of an orthotopic inoculation of the tumor cells and the isolation of highly-metastatic ones from the ascites of immunodeficient mice. To assess the biological differences between HSC-60 and 60As6 cells, we compared gene expression profiles for both cell lines by microarray analysis. Down-regulation of three gastric pit cell differentiation markers and some cytokeratins were found in 60As6. By contrast, several stem cell markers were up-regulated in this cell line. The expression profile suggests that 60As6 has a strong mensencymal phenotype, which is a characteristic of epithelial cell-derived CSCs.