Background: MUM1, also known as interferon regulatory factor 4 (IRF4), is a 51.6KDa nuclear transcription factor required for the development and activation of B lymphocytes. MUM-1 antibody is expressed in the nucleus and cytoplasm of plasma cells, as well as in a small subset of B cells located in the bright zone of the germinal center, as well as labeling central cells and their progeny, plasma cells, activated T cells, and a wide range of hemolymphoid tumors derived from these cells. At present, this antibody can be used as a tool for the identification and subclassification of lymphoid malignancies.