PRTN3 and rheumatoid arthritis: Levels of citrullinated proteins are significantly elevated at sites of inflammation including the joints of patients with rheumatoid arthritis (RA).1, 2 Whereas citrullination is ubiquitous in normal physiology and inflammation, anticitrullinated protein antibodies (ACPAs) are well established markers for the diagnosis of RA.3, 4 Appearing before evident symptoms, these autoantibodies correlate with poor prognosis and progressive joint destruction,5–8 and ACPA-positive patients often require more aggressive treatment.9