The presence of high (T) producer allele of IFNG(+874) gene that encodes the production of IFNG, the strongest IDO inducer, might augment the risk of IFN-alpha – associated depression, especially in patients with deficient serotonin formation of TRP, by additional decrease of TRP availability as a substrate for serotonin biosynthesis. This evidence concerns the gene IDO1 and depressive disorder.