Interestingly, some of these correlations are of negative sign when non-RA patients are analyzed: Anti-CD26 IgG levels correlate with TJC (Spearman R = − 0.419, p = 0.003) and SJC (Spearman R = − 0.306, p = 0.037), while Anti-CD26 IgM levels correlate with DAS28 (Spearman R = − 0.320, p = 0.032) and TJC (Spearman R = 0.430, p = 0.003). The gene discussed is DPP4; the disease is rheumatoid arthritis.