B2M and neoplasm: However, MMR-d colon cancers frequently lose HLA-class-I-mediated antigen presentation due to silencing of HLA class I genes, inactivating mutations in β2-microglobulin (encoded by B2M) or other defects in the antigen processing machinery13–16, which can render these tumours resistant to CD8+ T-cell-mediated immunity3–5,17.