HAAO and psychiatric disorder: Tryptophan is an essential amino acid obtained exclusively through diet, with its metabolites serving critical functions as neurotransmitters and signaling molecules.45 Activation of IDO1 leads to the accumulation of Kyn and its derivatives, including the neurotoxic metabolite QA.46 Elevated levels of QA and an increased Kyn/TRP ratio are commonly observed in psychiatric disorders.47 Our study demonstrated that anxiety in mice increased IDO1 and HAAO expression, while Tph1 was inversely correlated with Ido1 levels.