Monoclonal gammopathy of undetermined significance (MGUS) is a condition in which an abnormal protein — known as monoclonal protein or M protein — is in your blood. The protein is produced in a type of white blood cell (plasma cells) in your bone marrow.

MGUS usually causes no problems. But sometimes it can progress over years to other disorders, including some forms of blood cancer.

It's important to have regular checkups to closely monitor monoclonal gammopathy so that if it does progress, you get earlier treatment. If there's no disease progression, MGUS doesn't require treatment.

Monoclonal gammopathy of undetermined significance (MGUS) care at Mayo Clinic


People with monoclonal gammopathy generally don't experience signs or symptoms. Some people may experience a rash or nerve problems, such as numbness or tingling. MGUS is usually detected by chance when you have a blood test for another condition.


MGUS occurs when abnormal plasma cells produce M protein. The protein isn't harmful for most people. But if too much M protein accumulates, it crowds out healthy cells in your bone marrow and can damage other tissues in your body.

The precise cause of accumulating M protein isn't known. Genetic changes and environmental triggers 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 African-Americans are more likely to experience MGUS than are white people. The incidence in other races isn't clear.
  • Sex. MGUS is more common in men.
  • Family history. You may have a higher risk of MGUS if other people in your family have the condition.


Each year about 1 percent of people with MGUS go on to develop a more-serious disorder. The conditions that can develop depend on the type of MGUS you have:

  • Non-IgM MGUS (IgG, IgA or IgD MGUS). This is the most common type of MGUS and has the potential to progress to multiple myeloma. Less frequently, people with Non-IgM MGUS can progress to other disorders, such as immunoglobulin light chain (AL) amyloidosis or light chain deposition disease.
  • IgM MGUS. About 15 percent of people diagnosed with MGUS have IgM MGUS. It can progress to a certain rare cancer (Waldenstrom macroglobulinemia) and, less often, to lymphoma, AL amyloidosis or multiple myeloma.
  • Light chain MGUS (LC-MGUS). This newly classified type of MGUS can progress to a condition in which a certain type of M protein is in your urine (Bence Jones proteinuria) and to light chain multiple myeloma, AL amyloidosis or light chain deposition disease.

To determine your risk of progression, your doctor will consider:

  • The size and type of M proteins in your blood.
  • The amount of another small protein (free light chain) in your blood.
  • Your age at diagnosis. Your risk of developing a more serious condition increases the longer you've had MGUS.

Other complications associated with MGUS include bone fractures, blood clots and kidney problems.