Abstract
Monoclonal gammopathy of undetermined significance (MGUS) and smoldering multiple myeloma (SMM) are asymptomatic, pre-malignant disorders characterized by monoclonal plasma cell proliferation in the bone marrow and absence of end-organ damage. Recent advances in the pathogenesis, natural history, and prognosis of MGUS and SMM are reviewed. A new risk stratification system to determine the prognosis of MGUS using the size and type of M protein, and the serum free light chain assay is discussed.