MDSC are present in many cancers, which block adaptive immunity by inhibiting the activation of CD4+ and CD8+ T cells [40] and innate immunity by inhibiting natural killer cells [41].Once a tumor is established, tumor cell production of COX2 would maintain elevated MDSC levels, further blocking tumor immunity and allowing the malignant cells to proliferate without interference from the host’s immune system. This evidence concerns the gene PTGS2 and cancer.