IRF9 is a member of the IRF family of transcription factors and plays an important role in IFN signaling pathways and anti-viral immune responses. Type I and type III interferon stimulation induces phosphorylation and heterodimerization of STAT1 and STAT2. The activated STAT1:STAT2 heterodimer recruits IRF9 and forms a heterotrimeric complex termed IFN-stimulated gene factor 3 (ISGF3). The ISGF3 complex, in turn, translocates to the nucleus and transactivates IFN-inducible genes. The IRF9 protein in the ISGF3 complex is responsible for the recognition of IFN-stimulated response elements (ISRE). Mice lacking IRF9 fail to induce the expression of IFN regulated genes in response to type I interferon stimulation, indicating that IRF9 is crucial in mediating interferon signaling.