PRTN3 and rheumatoid arthritis: The inclusion of measurement of rheumatoid factor (RF) and anti-citrullinated peptide (ACPA) antibodies, generally composite RF (cRF) and ACPA-IgG, in the 2010 ACR/EULAR RA classification criteria is an indication of the sero-diagnostic utility of measurement of these auto-antibodies in RA1 and is well established worldwide2–5.