IDO1 and Huntington disease: IDO1, a rate-limiting enzyme in the kynurenine pathway, was reported to be upregulated and have increased activity in the striatum of an HD mouse model.49 IDO1 levels were significantly decreased in late HD compared with controls (P = 0.020), and showed moderate to strong correlations with CAP score (ρ = −0.45), TFC (ρ = 0.43) and VF (ρ = 0.50).