MUM1 (multiple myeloma oncogene-1, also called IRF4), a member of the IRF family transcriptional factors, is induced by antigen receptor mediated stimuli and plays a crucial role in cell proliferation, differentiation and survival. In the hematolymphoid system, MUM1 is primarily expressed in B- cells and actived T-lymphoid cells. In B-cells it is expressed on a small subset of germinal center (GC) cells committed to plasmacytic or memory cell differentiation in the “light zone” and in plasma cells. MUM1 has been identified as a marker of non-germinal center- derived DLBCL, a subtype also associated with more aggressive clinical behavior and poor prognosis, but absent in mantle cell lymphoma (pre-GC B-cells) and in follicular lymphoma (GC B-cells). MUM1 may be a potential histo-genetic marker for B-cell lymphomas. Additionally, MUM1 is a useful marker for Reed- Sternberg (HRS) cells in Hodgkin’s lymphoma.