MDSCs regulate the metabolism of L-arginine via inducible nitric oxide synthase (INOS) and arginase-1 (ARG1), which depletes the microenvironment of L-arginine, inhibits T cell proliferation, cytokine production, and expression of the T cell receptor CD3 zeta chain, converts L-arginine into polyamines, and promotes tumor growth. The gene discussed is ARG1; the disease is neoplasm.