IRF1 is the first identified member of the interferon regulatory transcription factor family. Its expression is induced in response to pathogen infection or the stimulation of several cytokines, including type I and II IFN, IL-1, IL-6, and TNF-α. Upon stimulation, IRF1 is transcriptionally activated by STAT1 and NFκB transcription factors via the GAS (interferon-gamma-activated sequence) element in the IRF1 promoter. IRF1 plays important roles in mediating immune response during infection and development of T cells and NK cells. In addition to the immunomodulatory functions, IRF1 has also shown to be involved in the regulation of cell cycle progression, tumor suppression, and apoptosis. IRF1-mediated transcription can be repressed by IRF2 through competing for the same binding region on the promoters.