We recently reported that ARG1 has a hierarchical role in generating an immunosuppressive tumor microenvironment among the L-arginine metabolizing enzymes; indeed, myeloid cells expressing high levels of the inducible nitric oxide synthase (iNOS) enzyme (i.e. TNFα- and NO-producing dendritic cells) actively sustained anti-tumor T cells response and were counteracted by ARG1 activation in tumor-associated macrophages [42]. The gene discussed is ARG1; the disease is neoplasm.