Its key rate-limiting enzymes, indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase (TDO), are highly expressed in various tumors, leading to tryptophan depletion and the accumulation of metabolites such as kynurenine in the tumor microenvironment (133). This evidence concerns the gene TDO2 and neoplasm.