Tumors with defects in MMR genes or microsatellite instability (MSI) often have an enhanced antitumor immune response, displaying a higher density of tumor-infiltrating lymphocytes (TILs).340,341 This phenomenon is attributed to high rates of mutations and increased levels of neoantigens in MMR-deficient tumors, which occur through different mechanisms, including mutant peptides, frameshift mutations, and indels in coding microsatellites.342,343 These neoantigens are presented on the cell surface by MHCI molecules, facilitating T-cell-mediated tumor cell killing (Fig. 5). The gene discussed is MRC1; the disease is neoplasm.