Through its capacity to act as an immunometabolic enzyme that propagates a state of tumor-promoting inflammation both by enabling tumoral immune escape and sustaining tumor neovascularization, IDO1 epitomizes a quintessential, non-oncogenic tumor promoter, integrating nearly all the cancer ‘hallmarks’ that involve interfacing with the TME rather than being tumor cell-intrinsic (51). This evidence concerns the gene IDO1 and neoplasm.