Tryptophan-2,3-dioxygenase (TDO), the rate-limiting enzyme of the kynurenine pathway, was observed in the high expression of AD brain (Yamada et al., 2009) and able to lead to the up-regulation of the kynurenine and the accumulation of quinolinic acid, which were neurotoxic in the development of AD (Guillemin et al., 2005). Here, TDO2 is linked to Alzheimer disease.