Glioblastomas are notable for high expression of the tryptophan catabolising enzymes indole-2,3-dioxygenase-1 (IDO-1) and tryptophan-2,3-dioxygenase-2 (TDO2); moreover, high expression of these enzymes strongly correlates with glioma severity [15–17] and poor patient survival [15, 16, 18–20]. Here, IDO1 is linked to glioblastoma.