IDO1 and nasopharyngeal carcinoma: In the present study, we aimed to investigate (i) the capacity of peripheral blood mononuclear cells (PBMC) to express IDO1 and IDO2, (ii) the plasma levels of Trp, Kyn, and the Kyn to Trp ratio (Kyn/Trp ratio), (iii) the correlation of plasma Kyn/Trp ratio with IDO gene expression in PBMC, (iv) the correlation of IDO gene expression and plasma Kyn/Trp ratio with clinicopathological parameters and patient survival, and (v) the correlation of interferon gamma (IFN‐γ), tumor necrosis factor‐α (TNF‐α), interleukin (IL)‐6, IL‐10 and IL‐17 cytokine levels with IDO in NPC.