SPP1 and hepatocellular carcinoma: We observed a significant increase of the expression of Pcna (gene of proliferating cell nuclear antigen), Ccna2 (gene of cyclin A2), Cdk1 (gene of cyclin-dependent kinase 1), Ssp1 (gene of secreted phosphoprotein-1, OPN), and Afp (gene of α-feto protein) in the double knockout mice, indicating that they might be more susceptible to development of HCC than Mdr2−/− mice.