Immunostaining of Ki-67 in HepG2-mock (pUSE-9 and pUSE-12) and HepG2-MEK1 (H-MEK1-15 and H-MEK1-17) tumors. pUSE-9, pUSE-12 H-MEK1-15 and H-MEK1-17 clones were subcutaneously injected on both flanks of male SCID mice as described under Experimental Procedures. Tumors were harvested, fixed, paraffin embedded and immunohistochemical analysis was performed as described under Experimental Procedures. The sections were stained with mouse anti-Ki-67 antibody (A). For each clone, 500 cells were counted in randomly chosen fields at 400 × magnification. The Ki-67 labelling index was expressed as the number of clearly labelled Ki-67 reactive nuclei in 500 cells counted (B). Differences in Ki-67 labelling index between HepG2-mock and HepG2-MEK1-tumours were statistically significant at p < 0.05 as analyzed by ANOVA test. Original magnification × 400.