Based on the presence of microsatellite instability (MSI-H) and/or the absence of protein expression for human mutL homolog 1 (hMLH1) or the absence of microsatellite instability (MSS/MSI-L), the tumors were divided into two groups: defective DNA mismatch repair (dMMR) tumors (poor differentiation tumor) and proficient DNA mismatch repair (pMMR) tumors. The gene discussed is MLH1; the disease is neoplasm.