E2F1 and hepatocellular carcinoma: First, SK-Hep1 hepatoma cells with forced expression of ISX or E2F1 showed increased cell proliferation activity from that in mock-transfected cells as assessed by cell counting at 72 h after cultivation (1.4- and 1.2-fold, respectively) and a bromodeoxyuridine (BrdU) incorporation assay (1.6-fold and 1.4-fold, respectively) (Figure 5A and 5B).