Defects in DNA mismatch repair (MMR) are responsible for a few cases of CRC, and the defect mainly involves MLH1 and MSH2, which leads to microsatellite instability (MSI). Finding MMR deficiency is important for predicting outcomes, screening for hereditary conditions like Lynch syndrome, and choosing treatments such as immune checkpoint inhibitors. Here, MSH2 is linked to mismatch repair cancer syndrome 1.