PRTN3 and Arthritis: As histone H4 had the highest frequency of appearance in ACPA IC (89%), we selected histone 4 for detailed peptide characterization as described in the Methods. Synovial fluids from 34 ACPA positive RA and 24 ACPA negative arthritis patients were tested for reactivity against the anti-citrullinated H4 peptides immobilized on microplates.