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Understanding MGUS and Smoldering Myeloma

This booklet contains information about monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM). Neither is an active disease state, but both are precursor states to active myeloma. It is therefore important to understand if, when, and how active myeloma might evolve and what monitoring and/or interventions are appropriate.


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MGUS (pronounced “EM-gus”) is the acronym for Monoclonal Gammopathy of Undetermined Significance, a term coined in 1978 by Professor Emeritus Robert A. Kyle of the Mayo Clinic in Rochester, Minnesota. The term MGUS describes a benign and asymptomatic condition that is characterized by an excess of a type of blood protein (“monoclonal protein”) made by immune system cells called plasma cells. MGUS is not cancer or a disease.

Smoldering multiple myeloma (SMM), a term coined by Professor Emeritus Philip Greipp of the Mayo Clinic in 1980, describes an asymptomatic intermediate stage between MGUS and active myeloma. It reflects a higher level of plasma cells in the bone marrow and a higher level of monoclonal protein in the blood than does MGUS. Like MGUS, SMM causes no damage to the kidneys, red blood cells, or bones. In other words, it causes no CRAB criteria.

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