For example, indoleamine 2,3-dioxygenase 1 (IDO1), which is the key enzyme that catalyzes the conversion of Trp to Kyn, is overexpressed in some types of cancer and interferon-γ (IFN-γ)-stimulated cells [2,3,4,5,6,7,8,9,10,11,12,14,15,18,19]. Here, IDO1 is linked to cancer.