Alterations in transcriptional control may contribute directly to carcinogenesis. Using the differential display technique in prostate cancer cells compared to normal prostate epithelial cells, we identified a down-regulated gene and an up-regulated gene in cancer cells. The down-regulated gene encodes human epithelial tropomyosin (TMe1), a member of the family of actin filament-binding proteins. The up-regulated gene encodes cytochrome c oxidase subunit VIc (COSVIc), a protein of the respiration chain in the mitochondrial inner membrane. The differential display pattern was confirmed by Northern hybridization in both prostate tissue and cell lines. In situ hybridization of malignant prostate epithelial tissue using a digoxigenin-labeled antisense riboprobe detected strong staining for mRNA of COSVIc, as opposed to very weak staining in normal prostate epithelium. The expression pattern of COSVIc may be a useful marker for studying the alteration of energy metabolism in cancer cells and for the diagnosis of prostate cancer.