In the kynurenine pathway, upregulated tryptophan 2,3-dioxygenase (TDO) in the AD gut leads to accumulation of neurotoxic quinolinic acid, which activates NMDA receptors and induces oxidative stress (hippocampal lipid peroxidation ↑ 48%). The gene discussed is TDO2; the disease is Alzheimer disease.