However, there are multiple enzymes regulated in the tryptophan metabolic pathway, including IDO, TDO, KMO, KAT, KYUN, etc. Although there are a large number of preclinical studies on the treatment of neurological and psychiatric disorders by regulating TRP metabolism, most of them focus on the metabolite levels of 5-HT, TRP, and KYN, the expression of IDO, etc., and fewer studies on complex enzymes. This evidence concerns the gene TDO2 and psychiatric disorder.