{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"Postmeiotic Segregation Increased 2 (PMS2) is a protein involved in DNA mismatch repair (MMR), critical for fidelity of DNA replication and genomic stability. PMS2 and MutL homolog 1 (MLH1) form a heterodimer protein complex called MutL⍺. MutL⍺ acts as an endonuclease to nick the DNA strand near the mismatch, creating a starting point for the removal and replacement of the incorrect DNA sequence. Defective DNA mismatch repair (MMR), due to PMS2 deficiency, leads to the accumulation of numerous replication errors, particularly in short nucleotide repeat sequences (microsatellites). This accumulation of errors, resulting in microsatellite instability-high (MSI-H), is the molecular basis of the replication error positive phenotype seen in hereditary non-polyposis colorectal cancer (HNPCC or Lynch syndrome)."}]}]}