To identify the primary pathway involved in the immunosuppressive activity of MDSCs induced by PIWIL1-overexpressing tumors, we first supplemented the Arginase-1 substrate l-arginine, or the iNOS inhibitor aminoguanidine, to the co-culture of stimulated T cells and MDSCs treated with conditioned medium derived from wild type and PIWIL1-overexpressing HCC cells. The gene discussed is PIWIL1; the disease is hepatocellular carcinoma.