The central dysregulation of the KP also seems to manifest in the blood stream, since accumulated serum QUINA levels [17] and serum Kynurenine to Tryptophan (KYN/TRP) ratios [14] (a surrogate marker of IDO1 activity) were observed in AD patients and women with high cortical amyloid-β burden [11, 18]. Here, IDO1 is linked to Alzheimer disease.