MUM1, also known as interferon regulatory factor 4, with a molecular weight of 50 kDa, is a nuclear transcription factor necessary for the development and activation of B lymphocytes. It can regulate the proliferation and differentiation of B cells and activate T lymphocytes. It is mainly used for the study of B-cell lymphoma. It is expressed in multiple myeloma, lymphoplasmacytic lymphoma and diffuse large B-cell lymphoma. It can be used as a tissue derived immunophenotypic marker of B-cell lymphoma. At the same time, it is also expressed in RS cells in anaplastic large cell lymphoma, peripheral T-cell lymphoma and classical Hodgkin's lymphoma. It is an immunophenotypic marker of malignant lymphoma, only limited to lymphocytes and melanocytes.