PRTN3 and rheumatoid arthritis: In addition to classical autoantibody ‘rheumatoid factor', anti-citrullinated peptide antibodies (ACPAs) are involved in the disease and have high diagnostic and predictive value.2, 3 ACPA is more specific for RA than rheumatoid factor, and is associated with the more severe disease phenotype of more frequent extra-articular manifestation4 and joint destruction.5