MDSCs are induced by various tumor-derived factors in the microenvironment, mainly granulocyte-macrophage colony-stimulating factor (GM-CSF), VEGF and IL-6 [81], and modulate the inflammatory microenvironment via depletion of many amino acids (such as L-arginin, L-tryptophan and L-cystein) [82–84], via increased production of nitric oxid (NO), ROS, inducible NO synthase (iNOS) and arginase-1 [85–87], and via expression of programmed death receptor ligand 1 (PD-L1), which ultimately inhibits T cell activation and proliferation and causes T cell apoptosis [88]. The gene discussed is NOS2; the disease is neoplasm.