In genetically susceptible individuals, the first stage of development of RA consists of accelerated citrullination of proteins in extra-articular sites, e.g., due to smoking or infection, including formation of rheumatoid factor (RF), anti-citrullinated protein antibodies (ACPA), and anti-carbamylated proteins (a-CarP) [3–6]. Here, PRTN3 is linked to rheumatoid arthritis.