Microsatellite instability (or MSI) is a type of mutation that occurs in small DNA segments. It involves repetitive nucleotides such as a long chain of repeating adenine nucleotides, or repeating cytosine-adenine dinucleotide sequences. During DNA replication, these repetitive sequences may decrease or increase in number, which is termed microsatellite instability. Ordinarily, DNA mismatch repair (MMR) genes code for proteins such as MLH1, MLH2, MLH3, MLH6, and PMS2 that maintain microsatellite stability. In some patients with colorectal cancer, mutations in MMR genes and proteins prevent errors in the length of the nucleoside chains from being repaired. These errors may persist in the tumor cells, eventually leading to the production of nonfunctional cellular proteins that promote tumorigenesis. This phenomenon occurs more frequently in patients with right-sided colon cancers.