IDO1 and cancer: Serum concentrations of tryptophan (Trp), kynurenine (Kyn) and Kyn/Trp ratio has been used as a surrogate of IDO-1 expression and several studies have shown that the Kyn/Trp ratio increases in the serum or plasma of patients with cancers, suggesting that enhanced IDO-1 activity may play a role in immunesuppression observed in cancer patients (37).