Fig. 4. Telencephalon-specific Rb deficiency induces ectopic S phase entry. (A) Females at E15.5 were injected with BrdU and, 2 h later, mutant and control embryos were removed and assayed for BrdU incorporation immunohistochemically. The conditional mutants exhibited extensive BrdU incorporation outside the ventricular zone (horizontal). (B) Quantification revealed that BrdU-positive cells in the control and mutant did not differ within the ventricular zone (VZ), with 48.8 ± 1.6 and 48.8 ± 2.9 positive cells/section, respectively. In the intermediate zone (IZ) and cortical plate (CP), there were only 14.0 ± 1.2 BrdU-positive cells in the control, as compared with 72.1 ± 1.8 positive cells in the mutant. Quantification was performed by counting cells within a superimposed grid (n = 8). Error bars denote standard error. Bar = 50 µm.