Cover legend: Cerebral cortex from a postnatal day 1 mouse. Neurons born on embryonic day 11. 5 (E11.5) were labeled by a single BrdU pulse (blue). Subcerebral projection neurons were labeled by injecting the retrograde tracer cholera toxin b subunit (green) into the cerebral peduncles at E17. Deep-layer neurons, which express the transcription factor CTIP2, are magenta. For more information, see the article by McKenna et al. in this issue (pages 549–564).