TDO2 and central nervous system cancer: The enzymatic degradation of tryptophan (Trp) to kynurenine (Kyn), mediated by indoleamine-2,3-dioxygenase-1 (IDO1) or tryptophan-2,3-dioxygenase (TDO2), is a key immunosuppressive pathway operative in gliomas and other types of tumors (4–8), and an emerging drug target in cancer immunotherapy (9–11).