MUM1 is one of nuclear transcription factors necessary for development and activation of B lymphocytes. 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. 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 MUM1 gene among T-cell lymphomas is observed. MUM1 is useful in a panel with other markers for subclassification of malignant lymphomas and identification of plasma cell differentiation. Particularly MUM1 may be useful for the identification of plasma cell differentiation when morphologic evidence is lacking and Ig light chains are difficult to interpret.