CXCL13 and synovial sarcoma: The serum CXCL13 level was increased in the HTLV‐1‐seronegative SS group compared to the non‐SS subjects, although the serum CXCL13 level of the HTLV‐1 carriers with SS was lower than that of the HTLV‐1‐seronegative SS patients (p = .07) (Figure 8B).