Monoclonal Gammopathy of Undetermined Significance (MGUS) is a condition in which plasma cells produce excess amounts of immunoglobulin protein. This occurs more frequently in people as they age. MGUS usually does not adversely affect a person's health, although a minority of people with MGUS will develop multiple myeloma. MGUS patients need to be monitored closely to detect a possible progression to multiple myeloma, but immediate treatment is not necessary.