Temporal and spatial combinations or codes of transcription factors control many aspects of development. Now, on p. , Helms and co-workers describe a code comprising bHLH transcription factors that controls the generation of early-born dorsal neuronal populations in mice. The dorsal spinal cord contains a diverse array of neurons that form the circuitry linking the sensory input from the periphery to spinal cord motoneurons and the brain. The researchers use fate mapping and genetic experiments to show that, while the bHLH transcription factor Mash1 is necessary and sufficient for the specification of dI3 and dI5 neurons, the number of these neurons that form depends on the activity of the bHLH transcription factor Ngn2. Thus, the researchers suggest, the balance of Mash1 and Ngn2 activity is pivotal in establishing the correct composition of neurons in the dorsal spinal cord.
