Mismatch repair deficient (dMMR)/microsatellite unstable cancers (MSI-H) are highly immunogenic, demonstrating one of the highest response rates to anti-PD-1/PD-L1 checkpoint inhibitor therapy across tumour types.1 Microsatellite instability occurs on the background of genetic and epigenetic alterations affecting the expression and/or function of one of the four mismatch repair proteins (MLH1/PMS2/MSH2/MSH6). The gene discussed is MSH2; the disease is neoplasm.