Prostaglandin E2 (PGE2) is a critical product of cyclooxygenase 2 (COX-2), which is overexpressed in most human cancers, affects tumor progression and immunosuppression, and stimulates arginase-1 (ARG-1) and nitric oxide synthase (NOS)-2 secretion from MDSCs24. Here, PTGS2 is linked to cancer.