PTGS2 and neoplasm: MDSCs are a heterogeneous population of myeloid cells including immature myeloid cells and myeloid progenitor cells and accumulate in tumor regions under the influence of various tumor-secreting factors including cyclooxygenase 2 (COX-2), granulocyte–macrophage colony-stimulating factor (GM-CSF), VEGF, TGF-β, and IL-6 [238].