Medullary breast cancer (MCB) is a morphologically and biologically distinct subtype that, despite cytologically highly malignant characteristics, has a favorable prognosis compared to the more common infiltrating ductal breast carcinoma. MCB metastasizes less frequently, which has been attributed to both immunological and endogenous cellular factors, although little is known about the distinct biology of MCB that may contribute to the improved outcome of MCB patients. To identify candidate genes, we performed gene array expression analysis of cell lines of MCB, ductal breast cancer and normal breast epithelia, and the differential expression of a panel of candidate genes was further validated by quantitative PCR and immunohistochemical analysis of cell lines and tumor biopsies. A limited number of genes, including several members of the GAGE and insulin growth factor binding protein (IGFBP) gene families, Vav1, monoglyceride lipase and NADP+-dependent malic enzyme, exhibited altered expression in MCB vs. ductal breast cancer, and the differences for some of these genes were confirmed on an extended panel of cell lines by quantitative PCR. Immunohistochemical analysis further established that the expression of monoglyceride lipase was restricted to ductal breast cancer and present in 77% of these tumors, while Vav1 was restricted to MCB and present in 60% of tumors. In this study, we have identified genes that are differentially expressed in MCB vs. ductal breast cancer and further analysis of the gene products should illuminate the biological differences between MCB and ductal breast cancer.