MyPathologyReport Printed: October 8, 2026

MUM1: Definition

MUM1 is a protein that helps control how certain immune cells mature. It is found in plasma cells, which make antibodies, and in B cells that are close to becoming plasma cells. Pathologists test for MUM1 to help classify lymphomas and plasma cell tumors. MUM1 is also called IRF4, the name of the gene that makes it.

MUM1 is a test result, not a diagnosis. It is one of the stains a pathologist uses to work out what type of cells are present and, for some lymphomas, which subtype is present. This article explains why pathologists test for MUM1 and what a positive or negative result means in your pathology report.

Where is MUM1 normally found?

MUM1 works inside the cell nucleus, switching other genes on and off. It helps B cells mature into plasma cells. In normal tissue, MUM1 is found in plasma cells, some late-stage B cells, and some activated T cells.

MUM1 stands for “multiple myeloma oncogene 1” because the gene was first discovered in a type of plasma cell cancer.

How do pathologists test for MUM1?

Pathologists test for MUM1 using immunohistochemistry (IHC). In this test, a thin slice of tissue on a glass slide is treated with an antibody that sticks to MUM1. A chemical reaction then colors any cell containing MUM1 brown so that it can be seen under the microscope.

Because MUM1 works in the nucleus, the brown color appears in the nucleus of positive cells.

Why do pathologists test for MUM1?

Pathologists test for MUM1 in the following situations:

To classify diffuse large B-cell lymphoma. Diffuse large B-cell lymphoma is divided into two main groups based on the type of normal B cell it most resembles. MUM1 is tested together with CD10 and BCL6 to place the lymphoma in one of these groups. The two groups can differ in how they behave and respond to treatment.
To identify plasma cell tumors. Plasma cell tumors, such as multiple myeloma and plasmacytoma, are positive for MUM1.
To help diagnose other lymphomas. MUM1 is positive in the Reed-Sternberg cells of classic Hodgkin lymphoma, in lymphoplasmacytic lymphoma, and in some other B-cell lymphomas.
To identify melanoma. Many melanomas are also MUM1-positive. This can be a helpful clue when the tumor type is unclear.

What does a positive MUM1 result mean?

A positive result means the tested cells contain MUM1. Your report may describe the result as “positive,” may give the percentage of tumor cells that are positive, or may say that MUM1 “is expressed.” In diffuse large B-cell lymphoma, MUM1 is read together with CD10 and BCL6. A positive MUM1 result often places the tumor in the non-germinal center or activated B-cell group.

In other tumors, a positive result supports a tumor made of plasma cells or related B cells. The pathologist always interprets MUM1 together with other stains.

What does a negative MUM1 result mean?

A negative result means the tested cells do not contain MUM1. Your report may describe the result as “negative” or “not expressed.” In diffuse large B-cell lymphoma, a negative MUM1 result, read with CD10 and BCL6, often places the tumor in the germinal center B-cell group.

Questions to ask your doctor

Why was MUM1 tested on my sample?
Was the MUM1 result positive or negative?
If I have diffuse large B-cell lymphoma, which subtype is it?
Does the subtype affect my treatment or outlook?
What other stains were done along with MUM1?
What is the final diagnosis in my report?