ACPA-positive and ACPA-negative RA have quite different pathogenesis6,7, and when future treatment targeting ACPA-positive RA specifically (e.g., PAD inhibitors) becomes available, protein arrays such as the one employed here may discriminate ACPA-positive and ACPA-negative RA better than the anti-CCP test. The gene discussed is PADI4; the disease is rheumatoid arthritis.