Interferon regulatory factor 4 (IRF4), also known as MUM1 and LSIRF, is a 51 kDa lymphocyte-restricted transcription factor. It is required for immunoglobulin class switching and terminal differentiation of B cells. IRF4 is overexpressed in multiple myeloma and cooperates with Myc in an autoregulatory loop. In T cells, IRF4 is required for the production of IL-4. IRF4 contains an N-terminal DNA binding domain that is homolgous to that in other IRF proteins. Within the C-terminal domain (aa 130 - 451), human IRF4 shares 90% aa sequence identity with mouse and rat IRF4. Alternate splicing may generate isoforms with N-terminal, C-terminal, or internal deletions.