ARG1 and neoplasm: Histone deacetylase inhibitors (HDACis) are important epigenetic regulators.265 Recent studies in preclinical mouse tumor models have shown that HDACis can significantly reduce ARG1, iNOS, and COX-2 expression in MDSCs, thus promoting the efficiency of immunotherapeutic agents.266,267 A study on EL4 lymphoma and LLC (Lewis lung carcinoma) mouse models reported that the selective class I HDACi entinostat reduced the immunosuppressive activity of PMN-MDSCs.