Furthermore, in accordance with a recent report by Hughes et al. (2012) of decreased tryptophan availability in a cohort of depressed patients, Maes and Colleagues (Maes et al., 2011; Maes and Rief, 2012) report that while depression and somatic disorders are both characterized by decreased tryptophan concentrations, there was no evidence in support of increased IDO activity in depression. Here, IDO1 is linked to major depressive disorder.