cTNC5 ACPA-positive sera were mostly found within the anti-CCP2 antibody-positive RA population with cTNC5 antibody levels highest in the anti-CCP2 antibody positive subgroup, as described for other ACPA.14 Of the patients with RA 4.9% within the anti-CCP2 antibody-negative group were also anti-cTNC5 ACPA-positive, demonstrating that not all ACPA-positive patients can be detected by testing for CCP reactivity. This evidence concerns the gene PRTN3 and rheumatoid arthritis.