Beta-2-microglobulin is the light chain that is common to HLA-A, HLA-B, HLA-C, and most MHC antigens.34 It is involved in immune system function and has been shown to increase in the presence of cytokines such as IFN-gamma, and IL-6.35 The level of B2M has been proposed as a biomarker of multiple diseases, including but not limited to multiple myeloma,36 prostate cancer,37 and viral infection.38 Beta-2-microglobulin has been investigated as a potential biomarker of MS activity (reviewed in ref. 39); however, results were conflicting, and its reliability was questionable. The gene discussed is B2M; the disease is myeloid sarcoma.