ARG1 and neoplasm: Nitric oxide synthase (NOS) and ARG-1 are two major Arg metabolizing enzymes, both of which play essential roles in the antitumor activity and immunosuppressive effects of DCs: high levels of NO can inhibit tumor growth by regulating DCs function; Arg metabolism indirectly affects the antitumor ability of DCs by regulating their maturation and functional status, and also by modulating T cell activation and proliferation [173].