MUM1 (multiple myeloma oncogene-1) is a 50 kDa proteinthat belongs to the interferon regulatory factor family oftranscription factors. It is often referred to as interferonregulatory factor 4 (IRF4). It is triggered by stimulimediated by antigen receptors and is crucial for cellsurvival, differentiation, and proliferation. The cytoplasmand nuclei of plasma cells, as well as a tiny proportion ofgerminal centre (GC) B-cells dedicated to plasmacytic ormemory cell differentiating in the “light zone,” express theMUM1 antibody. MUM1 is a great marker for Hodgkin’sand Reed-Sternberg cells in typical Hodgin’s disease andcan be used to subclassify lymphoid malignancies.