Genetic variation involved in DNA mismatch repair (MMR) pathway can lead to microsatellite instability (MSI), a specific type of high TMB tumors, and increased numbers of CD8+ tumor infiltrating lymphocytes (TILs), PD-1+TILs, and indoleamine 2,3-dioxygenase (IDO)+ tumor cells have been shown in MMR deficiency (dMMR) colorectal cancer [33]. The gene discussed is IDO2; the disease is neoplasm.