Beta-2-microglobulin (beta-2-m) is expressed on the surface of nucleated cells as a subunit of class I major histocompatibility complex molecules. Uncomplexed beta-2-m (11,800 daltons) is normally shed into the circulation in low concentration, freely filtered across glomerular basement membrane, and largely reabsorbed by proximal tubular epithelium. Increased serum/plasma concentrations of beta-2-m are frequently seen under conditions of increased cell turn over - including malignant neoplasms (especially B lymphocytes), inflammation, and some infections. Older studies of patients with multiple myeloma, before treatment, correlated high concentrations of beta-2-m with worse prognosis than observed in patients with myeloma and lower concentrations of beta-2-m. Elevated serum/plasma beta-2-m can be seen with decreased glomerular filtration rate/renal insufficiency. Elevated urine beta-2-m can be seen as a result of impaired proximal tubular reabsorption.