The serology of patients with Rheumatoid arthritis (RA) is characterised by persistently raised levels of autoantibodies of two main specificities, being those against Fc of IgG (Rheumatoid factors, RhF) and to peptide sequences on a number of different proteins which have undergone citrullination (anti-citrullinated protein/peptide antibodies - ACPA) [1]–[3]. The gene discussed is PRTN3; the disease is rheumatoid arthritis.