They are detectable in 50–70% of RA patients,1,2 and are included in the current classification criteria of RA.3, , –6 Autoantibodies can appear many years before disease onset, and both RF and ACPA have been associated with more severe disease, worse long-term outcomes,7, –9 and a higher rate of relapses after withdrawal of disease modifying antirheumatic drug (DMARD) therapy. Here, PRTN3 is linked to rheumatoid arthritis.