Induction of Er81 expression in proprioceptive afferents is controlled by peripheral NT-3 as axons reach the vicinity of target muscles, and thus occurs only approximately 3 d after proprioceptive neurons become post-mitotic [9,14]. This temporally delayed and target-induced upregulation of ETS transcription factor expression several days after a neuronal lineage of a specific identity first emerges is not restricted to DRG sensory neurons, but is also found in motor neuron pools [13]. Target-derived factors have also been implicated in controlling neuronal maturation of predetermined neurons in Drosophila, in which expression of members of the BMP family in the target region is essential for the induction of mature peptidergic properties in a subpopulation of neurons marked by the coordinate expression of the two transcription factors Apterous and Squeeze [11,12]. Thus, both in Drosophila and vertebrates, target-derived factors appear to act permissively to induce the expression of transcriptional programs involved in terminal neuronal maturation.