ARG1 and neoplasm: MDSCs are a subset of immature myeloid-lineage bone marrow cells in tumor tissue and are known for their T-cell suppressive activity by expressing arginase 1 (Arg1), transforming growth factor (TGF) β, inducible nitric oxide synthase (iNOS), reactive oxygen species (ROS), etc.4 Numerous lines of evidence clearly support the existence and function of MDSCs in human cancer patients and murine tumor models.5–7 Two major subpopulations of MDSCs, polymorphonuclear (PMN-, or granulocytic, G-) and monocytic (M-) types, are currently accepted.