To further dissect the involvement of TLR9 in T cell‐dependent versus T cell‐independent phases of inflammatory arthritis, we investigated disease development and progression in the K/BxN serum‐transfer model which is independent of T cells and characterized by a fast developing monophasic destructive arthritis in all tarsal joints.27 C57BL/6 mice lacking a functional TLR9 (TLR9−/−) and their littermate wild‐type controls (TLR9+/+) were injected with K/BxN serum to induce arthritis. The gene discussed is TLR9; the disease is arthritic joint disease.