NSD3 maintains IRF3 phosphorylation to enhance its transcriptional activity, promoting antiviral innate immune response. Transcription factor IRF3 is phosphorylated upon viral infection, and the phosphorylated IRF3 undergoes dimerization and nuclear translocation. In the nucleus, methyltransferase NSD3 directly methylates IRF3 at K366, and the NSD3-mediated IRF3 methylation maintains phosphorylation of IRF3 by preventing IRF3 dephosphorylation via disrupting the association of PP1 with IRF3.