Cover legend: Nectin-3 immunocytochemistry reveals the axons of neocortical callosal projection neurons (CPN; white) as they course through the corpus callosum early in development. In this issue, Molyneaux et al. report on the identification of a series of genes that individually and in combination define CPN and novel CPN subpopulations during embryonic and postnatal development. For more information, see the article by Molyneaux et al. in this issue (pages 12343–12354).