Transcription factor that plays an essential role in anti-viral immunity. It mediates signaling by type I IFNs (IFN-alpha and IFN-beta). Following type I IFN binding to cell surface receptors, Jak kinases (TYK2 and JAK1) are activated, leading to tyrosine phosphorylation of STAT1 and STAT2. IRF9/ISGF3G associates with the phosphorylated STAT1:STAT2 dimer to form a complex termed ISGF3 transcription factor, that enters the nucleus. ISGF3 binds to the IFN stimulated response element (ISRE) to activate the transcription of interferon stimulated genes, which drive the cell in an antiviral state. Interacts with STAT2 in the cytoplasm. Forms the interferon-stimulated gene factor 3 complex (ISGF3) with the heterodimer STAT1:STAT2; upon stimulation. By IFN-alpha and IFN-beta. Upon stimulation the regulatory phosphorylated alpha and beta subunits assemble with the gamma subunit and translocate from the cytoplasm to the nucleus. Belongs to the IRF family.