The remaining 20% of CRC tumors exhibit MSI due to defects in this DNA MMR pathway that corrects small base-pair mistakes in mononucleotide, dinucleotide, and trinucleotide repeat regions throughout the genome and are classified as having high or low MSI (MSI-H or MSI-L, respectively).5 A small fraction of MSI-H tumors result from germline mutations in 1 of 4 DNA MMR genes—MLH1, MSH2, MSH6, and PMS2—and result in the hereditary CRC condition called Lynch syndrome. Here, MLH1 is linked to Lynch syndrome.