It is well known that depression is associated with low 5‐HT function, while IDO is mainly activated by IFN‐γ, suggesting that the activation of IDO may be associated with depression (Brundin, Erhardt, Bryleva, Achtyes, & Postolache, 2015; Courtet et al., 2016; Miná et al., 2015). Here, IDO1 is linked to major depressive disorder.