Cancer cells are knownto actively inhibit immune recognition throughdiverse mechanisms, including loss of the antigen-presenting machineryor expression of inhibitory molecules and enzymes that induce T-cellsuppression.6 One specific enzyme is indoleamine2,3-dioxygenase (IDO), which catabolizes the amino acid tryptophan(Trp) to kynurenine (Kyn). Here, IDO1 is linked to cancer.