() encodes a transcription factor essential for the specification of layer 5 projection neurons (PNs) in the developing cerebral cortex. As with many developmental transcription factors, continues to be expressed into adulthood, suggesting it remains crucial to the maintenance of neuronal phenotypes. Despite the continued expression, a function has yet to be explored for in the PNs of the developed cortex. Here, we investigated the role of in mature neurons, using lentiviral-mediated delivery of a shRNA to conditionally knockdown the expression of in the mouse primary motor cortex (M1). RNA-sequencing analysis of -reduced M1 revealed significant changes to the transcriptome, identifying a regulatory role for in the mature M1. Kyoto Encyclopedia Genes and Genomes (KEGG) pathway analyses of -regulated genes indicated a role in neuronal signaling and plasticity, with significant enrichment of neuroactive ligand-receptor interaction, cell adhesion molecules and calcium signaling pathways. Gene Ontology analysis supported a functional role for -regulated genes in neuronal transmission and additionally indicated an importance in the regulation of behavior. Using the mammalian phenotype ontology database, we identified a significant overrepresentation of -regulated genes associated with specific behavior phenotypes, including associative learning, social interaction, locomotor activation and hyperactivity. These roles were distinct from that of -regulated genes identified in development, indicating a dynamic transition in function. Together our findings demonstrate a regulatory role for in the mature brain, with -regulated genes having functional roles in sustaining normal neuronal and behavioral phenotypes. These results support the hypothesis that developmental transcription factors are important for maintaining neuron transcriptomes and that disruption of their expression could contribute to the progression of disease phenotypes.