Monoclonal gammopathy of undetermined significance (MGUS) is a condition in which an atypical protein is found in the blood. The protein is called monoclonal protein or M protein.

This protein is made in the soft, blood-producing tissue in the center of bones. This blood-producing tissue is bone marrow. Monoclonal gammopathy of undetermined significance occurs most often in older men.

MGUS usually causes no problems. But sometimes it can lead to more-serious diseases. These include some forms of blood cancer.

People who have high amounts of this protein in the blood need regular checkups. That's so they can get earlier treatment if the condition gets worse. If it doesn't get worse, MGUS doesn't need treatment.


People with monoclonal gammopathy often don't have symptoms. Some people have a rash or nerve problems, such as numbness or tingling. A blood test for another condition might find MGUS by chance.


Experts don't know what causes MGUS. Changes in genes and being around certain chemicals, such as those used to kill pests, appear to play a role.

Risk factors

Factors that increase your risk of developing MGUS include:

  • Age. The average age at diagnosis is 70 years.
  • Race. Africans and Black Americans are more likely to get MGUS than white people are.
  • Sex. MGUS is more common in men.
  • Family history. Having family members with MGUS might increase the risk.


Each year, about 1% of people with MGUS get certain types of blood cancers or other serious diseases, such as:

  • Multiple myeloma.
  • Light chain amyloidosis.
  • Waldenstrom macroglobulinemia.
  • Lymphoma.

Other issues linked to MGUS include broken bones, blood clots, kidney problems, and damage to nerves outside of the brain and spinal cord, also known as peripheral neuropathy.