MRC1 and hereditary nonpolyposis colon cancer: Alterations in DNA produced during replication and recombination are repaired by the MMR system in an effort to maintain genomic stability, and tumours lacking MMR function exhibit a mutator phenotype.2 In addition, the cytotoxic effects of a number of alkylating agents used in the treatment of cancer are dependent on a functional MMR system.2 Hereditary non-polyposis colorectal cancer (HNPCC/Lynch syndrome) is an autosomal dominant condition, which comprises 2–5% of all colorectal cancers.