MUM-1/IRF4 (MUltiple Myeloma-1/Interferon Regulatory Factor 4 protein) is a myeloma-associated oncogene that is expressed late in the differentiation of post-germinal center B-cells. Antigenically stimulated B-cells that differentiate into plasma cells express MUM-1/IRF4. It has been identified as a partner gene involved in t(6;14)(p25;q32) translocation of multiple myeloma. This translocation results in juxtaposition of MUM-1 gene on 6p25 to the immunoglobulin heavy chain gene (IgH) locus on 14q32. Immunohistochemically, MUM-1 shows strong nuclear staining in myeloma cells. MUM-1 has been found to be a useful marker for identification of transition from germinal center B-cells (Bcl-6 positive, CD10 positive, MUM-1 negative) to post-germinal center cells such as immunoblasts and plasma cells (Bcl-6 negative, CD10 negative, MUM-1 positive, CD138 positive). Image courtesy of: Genevieve Crane, MD, PhD, Johns Hopkins Hospital, Baltimore, Maryland, USA.