Furthermore, tumor cells and tumor-infiltrating immune cells, such as dendritic cells (DCs) and macrophages, overexpress indoleamine 2,3-dioxygenase (IDO) for tryptophan metabolism and lead to kynurenine accumulation in the tumor microenvironment (TME), impeding T-cell activation and promoting immune escape11,12. This evidence concerns the gene IDO2 and neoplasm.