Notably, particular emphasis has been placed on indoleamine 2,3-dioxygenase (IDO), given its association with elevated levels of depression-like phenotypic markers in preclinical models (Andre et al., 2014; Dinel et al., 2014; O’Connor et al., 2009a; O’ Connor et al., 2009b). This evidence concerns the gene IDO2 and major depressive disorder.