The low levels of sTNF in SpA SF were also not related to an increase in the decoy receptors capturing sTNF as the soluble TNF receptor I (p55; SpA: median 2.45 ng/ml versus RA: median 10.3 ng/ml; P < 0.001) and receptor II (p75; SpA: median 17 ng/ml versus RA: median 31.15 ng/ml; P < 0.001) were also significantly decreased in SpA versus RA SF (Fig. 1, B and C), despite comparable synovial tissue mRNA levels in both conditions (Fig. S1, C and D). The gene discussed is TNFRSF1B; the disease is rheumatoid arthritis.