The anti-MUM1 antibody labels a 50 kDa protein, the multiple myeloma oncogene-1 (MUM1). MUM1 is encoded by the MUM1/IRF-4 gene, which has been mapped to 6q23-25 and identified as a myeloma-associated oncogene. It belongs to the interferon regulatory factor family of transcription factors and plays an important role in the regulation of gene expression in response to signals from interferon and other cytokines. MUM1-positive cells express the protein in the nucleus in a diffuse and microgranular pattern. However, some positivity is also observed in the cytoplasm of MUM1-expressing cells. In normal/reactive lymphoid tissues, such as lymph nodes, this antibody stains plasma cells, some B cells in the light zone of germinal centers and a subset of T cells (T cells in germinal centers and interfollicular areas). MUM1 expression has been described in diffuse large B-cell lymphoma (DLBCL). The anti-MUM1 antibody can stain other B-cell lymphomas such as lymphoplasmacytic lymphoma, chronic lymphocytic leukemia, follicular lymphoma, marginal zone lymphoma, lymphoblastic lymphoma/leukemia, primary effusion lymphoma, DLBCL, Burkitt-like lymphoma and classical Hodgkin’s lymphoma. However, the tumor cells in nodular lymphocyte-predominant Hodgkin’s lymphoma are negative or only weakly positive. MUM1 is also expressed in plasma cell myeloma.