In AD, indoleamine 2, 3-dioxygenase (IDO), the enzyme responsible for catabolising tryptophan into products that enter the KP, is stimulated through proinflammatory cytokine activity, including interferon-gamma (IFN-γ) [211], interleukin-12 (IL-12), interleukin-18 (IL-18) [212], and the Aβ 1–42 fragment [213]. This evidence concerns the gene IDO1 and Alzheimer disease.