IDO1 and neoplasm: IDO-1 in neoplastic cells, macrophages, and dendritic cells in neoplasia suppress T-cell proliferation and natural killer cells, promotes regulatory T-cell (Treg) and myeloid-derived suppressor cell (MDSC) development via tryptophan depletion and kynurenine production, and prevents the overactivation of the immune response [2–4].