Expression of the neuronal marker, MAP2, in the dorsal telencephalon at E12.5. MAP2 expression in coronal sections inBF1 (+/−) heterozygotes (A, D), NHAA rescue embryos (B, E), and BF1 WT rescue embryos (C, F) identifies differentiated cells of the preplate and cortical plate (mantle zone). The thickness of the mantle zone in the NHAA rescue is much greater than in the BF1heterozygote or in the BF1 WT rescue. The mantle zone:ventricular zone ratio is 0.15 in D, 1.4 inE, and 0.3 in F. The mantle zone is thinnest anteriorly and thicker posteriorly within the telencephalon. Therefore we show coronal sections from comparable planes at the level of the eye, in the heterozygote, the NHAA rescue, and theBF1 WT rescue. Scale bar: A–C, 400 μm;D–F, 50 μm.