Moreover, a recent study of AD patients reported significant positive correlations between serum and CSF concentrations of Kyn (r = 0.74), as well as between CSF levels of Tpr and 5-OH-Tpr [5-hydroxy-l-tryptophan (r = 0.74)], Kyn and KA (r = 0.79), and 5-OH-IAA and IAA [indole-3-acetic acid (r = 0.77)] [90]. Here, TPR is linked to Alzheimer disease.