MMR-deficient colorectal cancers represent approximately 13–15% of sporadic colorectal cancers resulting from MLH1 promoter hypermethylation, and in addition, there is a further 3% CRC due to Lynch syndrome, a genetic tumour predisposition syndrome conferred by inheritance of DNA variants affecting any one of the four MMR genes MLH1, MSH2, MSH6 and PMS2 [31,32,33,34,35]. The gene discussed is MRC1; the disease is BAP1-related tumor predisposition syndrome.