Tumor-derived factors and cellular components such as IL-10 and transforming growth factor-β (TGF-β), IDO, expression of negative co-stimulatory ligands PDL-1 and CTLA-4 and the presence of regulatory lymphocyte and myeloid cell populations pose challenges for the success of immunotherapy and anti-tumor responses [95,117]. The gene discussed is IDO1; the disease is neoplasm.