On serum collected from adult patients with ACPA+ RA (n = 32), ACPA− RA (n = 30), and matched healthy controls (n = 30), we used a highly multiplex autoantibody profiling assay to screen for over 1600 IgG autoantibodies that target full-length, correctly folded, native human proteins. The gene discussed is PRTN3; the disease is rheumatoid arthritis.