Furthermore, studies have shown that tumors with MMR defects can also contain other DNA repair defects, such as POLD or POLE mutations, and several immune checkpoint ligands, including PD-1, PD-L1, CTLA-4, LAG-3 and IDO, are also highly expressed in the tumor microenvironment of these patients (28, 29). This evidence concerns the gene IDO1 and neoplasm.