One of the most potent inducers of IDO-1 (in addition to the previously mentioned INF-γ) is IFN-α, which is used in the treatment of chronic hepatitis C. Several earlier studies reported a decrease in blood tryptophan concentrations and a concomitant increase of KYN which has been linked to IFN-α induced depression, suggesting the role of IDO-1 in the process [231,232,233,234]. Here, IDO1 is linked to major depressive disorder.