MLH1 and lymphoma: In order to recapitulate tumorigenesis with a hypermutable phenotype caused by MSI, knockout mouse strains have been generated for each of the Lynch syndrome genes (MSH2, MLH1, PMS2, and MSH6), although the principal type of tumor that develops in these mice is lymphomas and none of the heterozygous knockouts show a phenotype that is similar to that of Lynch syndrome patients [28–31].