MUM1 is a nuclear transcription factor necessary for developing and activating B lymphocytes. MUM1 belongs to the IRF gene family containing at least 10 widely expressed genes with similar DNA binding motifs, all involved in the regulation of cell growth, transformation and induction of apoptosis, and development of T-cell immune response. The synonym of MUM1 is Interferon Regulatory Factor 4 (IRF4). MUM1 is found mainly in B-cell lymphoma and melanocytic lesions. Significant variation in positivity, mainly due to chromosomal translocations involving the MUM1 gene among T-cell lymphomas, is observed. MUM1 is useful in a panel with other markers for subclassifying malignant lymphomas and identifying plasma cell differentiation. Particularly, MUM1 may be helpful in the identification of plasma cell differentiation when morphologic evidence is lacking, and Ig light chains are difficult to interpret.