(a) Transverse cryosection of E10.5 Venus::HES5 spinal cord. Venus::HES5 endogenous signal, OLIG2 - motorneuron progenitor marker, NGN2 - early marker of neuronal commitment; scale bar 20 μm. (b) Relative nuclear intensities of Venus::HES5 and NGN2 in motorneuron and interneuron progenitors; bars show mean and SD of at least 494 cells per domain from 5 slices in 2 experiments; Kruskal-Wallis with Dunn's multiple comparison test adjusted p-values **p=0.0032, ***p<0.001. 