Germline mutations in human mismatch repair genes (hMSH2, hMSH6, hMLH1, hPMS2) account for majority of the hereditary non-polyposis colorectal carcinoma (HPNCC). CpG dinucleotides in the hMSH2 and hMLH1 genes are hotspots for HNPCC mutations. These mutations cause a mismatch repair deficiency resulting in a mutator phenotype where the replication errors are not repaired. Microsatellites / simple repetitive sequences are prone to this type of replication errors and instability of these microsatellites correlates with the occurrence of HPNCC. hMSH2 binds to another MutS homolog protein GTBP to form a heterodimeric complex called hMutS beta, which binds to insertion/deletion loops in DNA.