We and others have recently found that PGE2 plays key roles in immune-related chronic inflammatory diseases in rodents and humans, for example, multiple sclerosis, rheumatoid arthritis, and inflammatory skin disorders, through promoting interferon-γ (IFN-γ)–producing T helper 1 (TH1) cells and interleukin-17 (IL-17)–producing TH17 cells by induction of key cytokine receptors IL-12Rβ2 and IL-23R, respectively (26–29). The gene discussed is IL17A; the disease is rheumatoid arthritis.