(a) Schematic shows area of interest (red box) in E16.5 mouse coronal section, as depicted in c,d,k,m,n,u. (b) Experimental outline of the proliferation and cell cycle exit assays. For labeling index, E14.5 and E16.5 control and mutant brains, harvested after a 1hr BrdU pulse, were processed for BrdU and DAPI staining (c,m). For quit fraction analysis, E16.5 control and mutant brains, pulsed with BrdU at E15.5, were processed for BrdU and Ki67 (k,u). Magnified view of DAPI-stained cortical nuclei shows differences in size and density between controls and mutants (d,n). (e-j,l) E14.5 and E16.5 H1047R mutants had similar labeling indices (BrdU+ cells/total DAPI+ cells); E16.5 H1047R mutant neocortex displayed reduced cell density (x105 DAPI+ cells/mm3 volume), larger nuclear and cell size (μm2) and similar quit fraction (BrdU+Ki67- cells/total BrdU+ cells). (o-q) E14.5 E545K mutant neocortex was similar to control in labeling index, cell density and nuclear size. (r-t,v) E16.5 E545K mutant showed significantly higher labeling index and quit fraction, reduced cell density, and enlarged cell and nuclear size, compared to controls. Data are represented as mean ± SEM (e,f,h,i,I,o,p,r,s,v) or as median-centered box-and-whisker plots (g,j,q,t); *p<0.05; **p<0.001; ***p<0.0001. Scale bars: 50 μm (c,d,m,n); 100 μm (k,u). See also Figure 3—figure supplements 1–2.