IDO1 and neoplasm: In addition, tumor cells can secrete suppressive factors (IL-10, TGF-β and IDO), express immune inhibitory molecules (FasL and PD-L1), directly inhibit tumor-specific T-cell expansion and proliferation, or induce T-cell apoptosis (Croci et al, 2007; Dong et al, 2002; Uyttenhove et al, 2003; Whiteside, 2008).