IDO1 and neoplasm: How low tryptophan concentrations reach in clinical IDO1-expressing human tumours is not well-documented, but tryptophan concentrations as low as 5 μM tryptophan, as used in our cultures, and which is sufficient to trigger a stress response in cancer cells [8], have been reported in mouse melanoma tumour models that are constitutively metabolising tryptophan [17].