We have reported that IL-17 levels in synovial fluids are significantly higher in patients with RA than in patients with osteoarthritis and that IL-17 stimulates osteoclast differentiation by inducing the expression of receptor activator of NF-κB ligand (RANKL) via a mechanism involving the synthesis of prostaglandin E2 in osteoblasts in vitro [21]. The gene discussed is IL17A; the disease is osteoarthritis.