We have demonstrated anti-CCP positivity in 94% and rheumatoid factor (RF) positivity in 97% of patients with RA-BR, which is significantly higher than would normally be seen in patients with RA alone.19 The mechanism is unknown, although RA-BR patients are frequently never smokers,20 hence a group that we would not expect to be at high risk of ACPA-positive RA. Here, PRTN3 is linked to rheumatoid arthritis.