Hepatoma cells respond to activin by up-regulating the expression of p21WAF1/Cip1, p16 and p15INK4B proteins that suppress the cyclinD-CDK4/6 and cyclinE-CDK2-mediated phosphorylation of the Rb protein [23,35,36]. The gene discussed is CDKN2A; the disease is hepatocellular carcinoma.