STAT1, also known as signal transduction and activator of transcription-1, is a ubiquitously expressed cytoplasmic protein that becomes phosphorylated in response to cytokine signaling (interferon alpha, interferon gamma, EGF, PDGF, and IL-6) by receptor-associated kinases and translocates to the nucleus to act as a transcriptional activator. Two isoforms of STAT1 exist as a result of alternative RNA processing with apparent molecular weights of 88 and 91 kD. STAT1 has been shown to interact with a number of proteins including STAT3, FAK, MCM3, MCM5, TRADD, BRCA1, KIT, IL-27 receptor, IL-2R beta and gamma, IFNalphaRbeta, STAT2, c-Src, and many others. STAT1 knockout mice are highly sensitive to infection with microbial infections and viruses. The SAT-84 monoclonal antibody recognizes human STAT1 and has been shown to be useful for Western blotting.