IDO1 and diabetes mellitus: In this study, we explored the clinical association of the Trp-Kyn metabolism via IDO1 induction in DM and demonstrated that (1) an increased serum Kyn/Trp ratio in DM correlated with disease activity; (2) Cox regression analyses identified ln(Kyn/Trp) as an independent prognostic predictor of mortality; and (3) serum Kyn/Trp ratio decreased upon treatment.