Tryptophan catabolism by IDO1 and TDO2 is critical within the tumour microenvironment (TME) as lowered tryptophan and raised kynurenine each confers an anti-immune response, by attenuating CD8+ Tcell infiltration, and promoting regulatory CD4+ cell differentiation [18, 21, 22]. This evidence concerns the gene CD4 and neoplasm.