During development, sensory neurons form neural circuits with motoneurons. Although the anatomical details of these circuits are well described, less is known about the molecular mechanisms underlying their formation. To investigate the involvement of motoneurons in sensory neuron development, Hirohide Takebayashi and colleagues analyse sensory neuron phenotypes in the dorsal root ganglia (DRG) of Olig2 knockout mouse embryos, which lack motoneurons (see p. 1125). These embryos, they report, also have reduced numbers of sensory neurons but increased numbers of apoptotic cells in the DRG. In addition, the axonal projections of the sensory neurons in these embryos are abnormal. Because neurotrophin 3 (Ntf3) and its receptors are strongly expressed in motoneurons and sensory neurons, respectively, the researchers also investigate whether Ntf3 is one of the motoneuron-derived factors that regulate sensory neuron development. Notably, the sensory neuron phenotypes in Ntf3 conditional knockout embryos resemble those observed in Olig2 knockout embryos. Thus, the researchers propose, motoneuron-derived Ntf3 is a pre-target neurotrophin that is essential for survival and axonal projection of sensory neurons.