Transcription factor involved in neuronal fate specification. First required in embryonic CNS development to define the number of cells that express apterous (ap) in the ap thoracic cluster of interneurons. Later on, it plays a central role in the combinatorial code of transcription factors that specifies the fate of the Tv neuron in the ap cluster by participating in the transcription regulation of FMRFa in Tv cells. Also required for projection neuron dendritic targeting 