IRF9 (interferon regulatory factor 9; also ISGF3-gamma) is a 48 kDa member of the IRF family of proteins. It is widely expressed, and serves as a component of the ISGF3 complex. Following activation of IFNAR by IFN-alpha/beta, IRF9 is acetylated by CBP on Lys81. IRF9 then associates with activated STAT1 and STAT2 to form an ISGF3 complex that is translocated into the nucleus. Human IRF9 is 393 amino acids (aa) in length. It contains an N-terminal DNA-binding region (aa 11 - 112) that contains an NLS (aa 66 - 85), plus a STAT-binding domain (aa 200 - 393). There are two potential isoform variants. One shows a deletion of aa 218 - 331 and 340 - 393, while another shows a five aa substitution for aa 217 - 393. Over aa 238 - 393, human IRF9 shares 79% aa identity with mouse IRF9.