The strongest evidence for IDO playing a causal role in LPS or bacille Calmette–Guréin induced depression is the finding that inflammation fails to induce depressed behaviors in mice lacking the indolamine 2,3 dixoxygenase gene or when an inhibitor of the enzyme is given (O’Connor et al., 2009a,b; Raison et al., 2010b). Here, IDO1 is linked to major depressive disorder.