Several potential mechanisms warrant more studies in future, such as specific differential immunosuppression, acute and chronic stress-related responses in different cytokines, and inflammatory activity between cytokines and enzyme indoleamine 2,3-dioxygenase, kynurenine, and tryptophan, which may be implicated in the cytokine-mediated pathogenesis of cancer and depression. This evidence concerns the gene IDO2 and depressive symptom measurement.