The antibody against these citrullinated proteins—ACPA—is detected in the sera of RA patients many years before clinically overt arthritis is present, indicating that ACPA may play an important role in RA pathogenesis.16 However, it remains unclear whether ACPA plays a causative, pathogenic role in RA pathogenesis or whether it is simply a bystander, resulting from joint inflammation. Here, PRTN3 is linked to arthritic joint disease.