Reduction of IL-2 levels in psychiatric disorders has been hypothesized to be a result of increase HPA axis activation via IDO (indoleamine 2,3-dioxygenase) mediated serotonin depletion and increased noradrenergic activity through the sympathetic nervous system (Gabbay et al., 2010; Himmerich et al., 2019). This evidence concerns the gene IDO1 and psychiatric disorder.