IDO1 and neoplasm: Regardless their subclassification, MDSCs play a critical role in tumour microenvironment (TME) by exerting immunosuppressive functions through the production of several factors including arginase 1 (Arg1), inducible nitric oxide synthase (iNOS), indoleamine 2,3-dioxygenase 1 (IDO1), TGFβ, IL-10, and ROS [124].