Multiple myeloma oncogene-1 (MUM-1) is a lymphocyte-specific member of the interferon regulatory factor family of transcription factors encoded by the MUM-1 gene. MUM-1 antibody is expressed in the nuclei and cytoplasm of plasma cells and a small fraction of B-cells located in the light zone of germinal centers. MUM-1 labels centrocytes and their progeny, plasma cells, activated T-cells and a wide spectrum of hematolymphoid neoplasms derived from these cells. MUM-1 has been reported to play an important role in mediating B-cell activation and differentiation. Therefore, this antibody may be used as a tool for the identification and the sub classification of lymphoid malignancies.