For example, Honda et al. used collagen‐induced arthritis, a model of RA, and found that deficiency in IP receptors or EP2/EP4 receptors significantly reduced the severity of arthritis, as assessed by synovial cell proliferation, inflammatory cell infiltration and joint destruction, which was accompanied by a significant reduction in the content of IL‐6 in arthritic paws (Honda et al., 2006). This evidence concerns the gene IL6 and rheumatoid arthritis.