Liver cancer is the fifth most common cancer worldwide and unlike certain other cancers, such as colon cancer, a mutational model has not yet been developed. We have performed gene expression profiling of normal and neoplastic livers in C3H/HeJ mice treated with diethylnitrosamine. Using oligonucleotide microarrays, we compared gene expression in liver tumors to three different states of the normal liver: quiescent adult, regenerating adult, and newborn. Although each comparison revealed hundreds of differentially expressed genes, only 22 genes were found to be deregulated in the tumors in all three comparisons. Three of these genes were examined in human hepatocellular carcinomas and were found to be upregulated. As a second method of analysis, we used Representational Difference Analysis (RDA) to clone mRNA fragments differentially expressed in liver tumors versus regenerating livers. We cloned several novel mRNAs that are differentially regulated in murine liver tumors. Here we report the sequence of a novel cDNA whose expression is upregulated in both murine and human hepatocellular carcinomas. Our results suggest that DEN-treated mice provide an excellent model for human hepatocellular carcinomas. Oncogene (2001) 20, 27042712.