Regrettably, extensive studies on the systemic effect of local IDO protein expression are lacking, except for a study in patients with diffuse large B cell lymphoma, which did not find an association between serum kynurenine level and IDO expression in the tumor (49) and a study in prostate cancer wherein a positive correlation between IDO1 mRNA in PT samples and the kyn/trp ratio in serum was observed (50). This evidence concerns the gene IDO1 and prostate cancer.