Likewise, cyclooxygenase-1 (COX-1)- and COX-2-derived prostanoids present in primary tumor-derived supernatants from several freshly isolated human tumor types block DC differentiation as well (33), and the source of these suppressive mediators may be not only tumor cells themselves but also stromal cells within the tumor microenvironment, as stromal cell-derived prostaglandin-E2 (PGE2) was recently shown to inhibit the differentiation of both bone marrow- and monocyte-derived DC (34). This evidence concerns the gene PTGS1 and neoplasm.