sentences:  Among 20 cancers evaluated, microsatellite instable colon cancer (mismatch repair deficient) had the second highest frequency of mutations (after melanoma).A heterodimer between MSH2 and MSH6 first recognizes the mismatch, although a heterodimer between MSH2 and MSH3 also can start the process.The formation of the MSH2-MSH6 heterodimer accommodates a second heterodimer of MLH1 and PMS2, although a heterodimer between MLH1 and either PMS3 or MLH3 can substitute for PMS2.