Recent advances have elucidated a role for the immune system, which is now recognized to be an instrumental component to cause depression from neurotoxicity rather than tryptophan depletion (Wichers et al., 2005), and although modest tryptophan decreases have been associated with depression, these occur independent of IDO activity (Wichers et al., 2005). Here, IDO1 is linked to depressive symptom measurement.