MUM1 belongs to the IRF gene family containing at least 10 widely expressed genes with similar DNA binding motif all involved in regulation of cell growth, transformation and induction of apoptosis as well as development of T-cell immune response. MUM1 plays an important role in the regulation of gene expression in response to interferon and other cytokines. MUM1 is a gene primarily identified in Multiple Myeloma cell line where it was localized in region of chromosomal translocations t (6;14) (p25;q32) placing this gene in proximity of IgH enhancer locus. Expression is also important for the differentiation of monocytes along macrophage and dendritic cell pathways. The expression of MUM1 protein appears at the later stages of B-cell differentiation after the expression of CD10 and Bcl-6. B-lymphocytes in light zone of germinal centers (late stage of germinal center differentiation) and post-germinal lymphocytes are generally positive. The expression of MUM1 and Bcl-6 in normal germinal center B cells appears to be mutually exclusive. MUM1 is constantly found at all stages of differentiation of plasma cells. Nuclear expression is present also in a subpopulation of activated T- lymphocytes. MUM1 protein is furthermore expressed in normal and neoplastic melanocytes but not found in other cell types. Some oncogenic viruses (HTLV-I and EBV), activate the NF-?B pathway and consequently elevate MUM1/IRF4 expression.