Dominant-transforming oncogenes are frequently detected in mouse liver tumors, but are rare or inconsistently detected in rat liver tumors. Most of those that have been identified are members of the ras family. While altered expression of many oncogenes has been reported, an increase in the expression of the c-myc gene is consistently observed in both rat and mouse hepatocellular tumors. Both hepatocytes and liver epithelial cells have been immortalized or transformed with viral or cellular oncogenes. Immortalization of cells occurs without the loss of differentiated functions, while transformation induces the expression of many genes/gene products associated with liver cancer in vivo. Cells transformed with chemical carcinogens or oncogenes display a phenotype of growth factor independence or greatly reduced growth factor requirements. Transformation is frequently associated with a substantial decrease in the expression of the exogenous growth factor receptor in the hepatocellular tumors.