To succeed in many walks of life, it is necessary to start at the bottom. As far as neuronal specification in the central nervous system is concerned, developmental neuroscientists seem to have taken this principle to heart. Over the last few years, the patterning of the ventral neural tube has received considerable attention, perhaps spurred on by the identification of the ventral-derived signalling molecule Sonic hedgehog around a decade ago. By contrast, dorsal neuronal identity was frequently dismissed as a default state that occurs in the absence of ventral signals, and it is only recently that dorsal neural patterning has really come to the fore. It is now becoming clear that the specification of neurons in the dorsal neural tube is controlled by a distinct set of patterning signals, and the importance of the roof plate as a source of these signals is increasingly appreciated. This progress is reflected in the review by Tamara Caspary and Kathryn Anderson on page 290 of this issue. They consider the molecular mechanisms that underlie the specification of dorsal interneurons in the spinal cord, and describe how the cellular domains that give rise to these neurons are initially patterned.

This review highlights the value of transgenic mouse models for understanding the processes that underlie the development of the nervous system, an approach that has also proved fruitful for studying the patterning of regions and circuits in the forebrain. This is the focus of the review by Guillermina López-Bendito and Zoltán Molnár on page 276. They describe how studies in knockout mice have helped in the identification of the signals that thalamic axons encounter as they grow towards the cortex, and they also discuss how these thalamocortical afferents might in turn impose regional information on the cortex.