While serum ACPA helped develop an important diagnostic tool for RA, current immunological views (21) suggest that the majority of citrullinated proteins represent neoantigens (instead of autoantigens) that have undergone extensive post-translational modification (citrullination) and then triggered the production of widely cross-reactive ACPAs, and likely T cell responses, in RA patients. Here, PRTN3 is linked to rheumatoid arthritis.