Increased PH-3-labeling in the cortices of Robo1−/−/Robo2−/− and Slit1−/− mice at E14.5. Histograms show number of PH-3+ apical and basal progenitor cells in Robo1−/−/Robo2−/− mice at E12.5 (A) and E15.5 (B), Slit single (Slit1−/−, Slit2−/−, and Slit3−/−) mutants (C) and Slit1−/−/Slit2−/− double mutants at E14.5 (D), compared with control littermates. A, At E12.5, histograms show statistically significant increases in apical progenitor cells in the MGE of Robo1−/−/Robo2−/− mice compared with control littermates. B, At E15.5, no significant increase in either progenitor cell population was observed. C, Similarly, no significant increases were found in Slit single mutants, with the exception of basal progenitors in the cortices of Slit1−/− mice. D, However, in Slit1−/−/Slit2−/− mice, an increase in apical and basal progenitors was observed in the cortex, LGE, and MGE; *p < 0.05, **p < 0.004.Ctx, Cortex.