Expression of IDO, an enzyme involved in tryptophan degradation pathway, was reportedly observed on both tumor cells and macrophages in tumor stroma of NPC. IDO metabolites, also secreted by EBV-infected cells, could inhibit NKG2D expression, and consequently, NKG2D-mediated cytotoxicity of NK cells. This evidence concerns the gene KLRK1 and nasopharyngeal carcinoma.