IDO1 and neoplasm: It has been identified that overexpression of IDO1 is an immunosuppressive feature in a range of tumor types by catalytically converting essential tryptophan (Trp) to kynurenine (Kyn), which is characterized as an immunosuppressive molecule to induce the acquired peripheral tolerance by causing cell cycle arrest and death of effector T cells and in meanwhile activate the immunosuppressive regulatory T cells (Tregs) [25–27].