Beta-2-microglobulin (B2M) is another indispensable element of antigen processing–presenting machinery (APM) that participates in the MHC class-I antigen presentation; its loss, particularly through the loss of heterozygosity, may lead to the subsequent loss of MHC Class 1 and the proper presentation of tumor antigens, which then hampers an effective anti-tumor response and contributes to immune evasion and resistance to therapy. The gene discussed is B2M; the disease is neoplasm.