PTGER2 and neoplasm: In addition, PTGER2 and PTGER4 are known to down-regulate the anti-tumor activity of PTGER-expressing natural killer cells, which are a type of cytotoxic T-lymphocytes (CTLs), by inhibiting their ability to kill tumor cells, cytokine synthesis, and chemotactic activity; thus, contributing to the pro-tumorigenic environment [167,168].