- Monoclonal gammopathy of undetermined significance (MGUS) and smoldering myeloma (SMM) are premalignant plasma cell proliferative disorders characterized by elevated monoclonal protein and bone marrow plasma cells. MGUS affects 3.2% of Caucasians over the age of 50 and has a 1% annual risk of progression to MM. approximately 3000 cases of SMM are diagnosed annually with a 10% annual risk of progression to MM.