Thus, the absence of correlations between the serum Kyn/Trp ratio and PD-L1 immunohistochemical expression might suggest the importance of an integrated evaluation of histological (PD-L1 and IDO1) and serum (soluble PD-L1 and Kyn/Trp ratio) markers for specifically targeted NSCLC immunotherapy. The gene discussed is IDO1; the disease is non-small cell lung carcinoma.