MUM1 is a nuclear transcriptional factor (IRF4 or Multiple Myeloma 1 ) and is expressed in final step of intragerminal center B cell differentiation and in post-germinal center B cells. MUM1 is usually mutually exclusive with BCL6 in nonneoplastic tissue. Nuclear expression is present also in a subpopulation of activated T- lymphocytes and expressed in normal and neoplastic melanocytes. In neoplasms MUM1 is found mainly in B-cell lymphoma and melanocytic lesions. In combination with CD138 and Ig´s makes MUM1 more specific marker for differentiating B-cells before plasma cell stage. MUM1 helps to divide diffuse large B cell lymphomas into germinal center (MUM1-) /non-germinal center (MUM1+) phenotypes and helps also to differentiate double hit from Burkitt and DLCL.