ARG1 and neoplasm: As a critical marker for the anti-inflammatory macrophage subset, arginase-1 competes with inducible nitric oxide synthase (iNOS) and hydrolyzes l-arginine into urea and ornithine, a precursor to l-proline and polyamines, which suppress NO-mediated cytotoxicity via l-arginine consumption, enhance collagen synthesis and fibrosis via l-ornithine formation, and increase cellular proliferation via polyamine generation, all important for macrophage-mediated tumor-promoting functions24, 25.