IDO1 and neoplasm: For example defects in T cell receptor signaling, tumor-induced impairment of antigen presentation, activation of negative co-stimulatory signals, such as CTLA-4/CD80 (or CTLA-4/CD86) and PD-1/PD-L1, elaboration of immunosuppressive factors (IL-10, TGF-β, galectin-1, gangliosides, and PGE2), inactivation of pro-apoptotic pathways (FasL, TRAIL, IDO, and RCAS1), inhibition of natural killer (NK) cell mediated cytotoxicity, and inhibition of differentiation and maturation of dendritic cell (DC) have been found to establish an immunosuppressive environment that promotes tumor growth (29).