For example, HPLC is used to determine the concentration of Trp and Kyn in human plasma/serum, such as assessing the activity of IDO before and after treatment in patients with early stage NSCLC via Kyn and the kynurenine to tryptophan ratio (Kyn/Trp) in human blood [88]. The gene discussed is IDO1; the disease is non-small cell lung carcinoma.