Cover legend: This image of an organotypic slice culture from mouse cortex at 4 d in vitro shows the presence of neurons (labeled by immunostaining against Tuj1, red), nuclei (Hoechst stain, blue), and transfected Sox4 (green). Sox4 may help specify intermediate progenitor cells in the developing cortex. For more information, see the article by Chen et al. (pages 10629–10642).