Expression of plasmids encoding constitutively active MEK1 or MEK2 drives sustained phosphorylation of ERK1 or ERK2, which, in turn, restricts the proliferation of human hepatocellular carcinoma cell line Huh-7 (81) and human normal intestinal epithelial crypt cell cultures (85). The gene discussed is MAP2K1; the disease is hepatocellular carcinoma.