MKI67 and neoplasm: Although levels of Ki67 and cleaved caspase-3, markers for proliferation and apoptosis, respectively, were similar (Fig. S4a, b), we observed a significant reduction in the expression of the cell cycle inhibitor p21 in PtenΔ/ΔBRF1Tg tumours (p = 0.0151) (Fig. 2h, i), consistent with the cell cycle data in Fig. 1g, h.