Interferon regulatory factor 4 (IRF4) is a member of the IRF family of transcription factors,expressed in most cell types of the immune system. IRF4 is a transcription factor essential for the development of T helper-2 (Th2) cells,IL17-producing Th17 cells,and IL9-producing Th9 cells,as well as dendritic cell (DC). It binds to and activates the interferon-stimulated response element (ISRE) of the MHC class I promoter,and may have a role in ISRE-targeted signal transduction mechanisms specific to lymphoid cells.