IDO1 and non-small cell lung carcinoma: However, besides its possible use as a prognostic parameter in advanced-stage patients, we believe in the potentialities of the serum Kyn/Trp ratio as a non-invasive biomarker in the follow-up of NSCLC to monitor over time the level of IDO1 activity and its impact on immune responses, and to choose a new IDO1-based target therapy in addition to the currently available ones.