Enhanced expression of IDO1 and TDO2 leads to greater catabolism of tryptophan leading to raised levels of the tryptophan metabolite kynurenine in the tumour, and thereby enhancing activation of the aryl hydrocarbon receptor (AhR) for which kynurenine is an endogenous ligand (see Fig. 1). This evidence concerns the gene IDO1 and neoplasm.