TDO2 and nervous system disorder: Recent studies have shown that inhibiting the activities of de novo pathway enzymes (Figure 2B), such as tryptophan-2,3-dioxygenase (TDO) (Breda et al., 2016) and kynurenine-3-monooxygenase (KMO) (Mole et al., 2016), may help alleviate specific neurological disorders.