MAPT and Alzheimer disease: The ratio of KYN/TRP was increased in the plasma and CSF of patients with AD and an increased 3-HK/KYN ratio in samples from CSF positively correlated with amounts of t-tau and p-tau peptides, while plasma KYN and PIC inversely correlated with p-tau and t-tau, respectively [186,187,188,189,190,191].