To dissect the role of TLR9 in the pathogenesis of RA, we induced arthritis in TLR9−/− mice and blocked TLR9 activation in PIA by applying suppressive ODNs which have previously been shown to block TLR9 on murine and human cells in vitro.20, 21 The data obtained demonstrate involvement of TLR9 in the T cell‐dependent phases of erosive arthritis, suggesting a pathogenic role of endogenous DNA and TLR9 in the initiation of arthritogenic autoimmune responses. The gene discussed is TLR9; the disease is rheumatoid arthritis.