MSH2 and cancer: The first subtype includes microsatellite instability (MSI) high cancers and possesses mutations or epigenetic alterations in mismatch repair (MMR) proteins MSH2 (MutS Homolog 2), MSH6 (MutS Homolog 6), MLH1 (MutL Homolog 1), or PMS2 (Postmeiotic Segregation increased 2).