Citrullination is involved in many physiological functions, including skin keratinization, neuron myelination, and formation of neutrophil extracellular traps (NETs) (1–3) but is also thought play a pathogenic role in anti-citrullinated protein-antibody (ACPA)-positive rheumatoid arthritis (RA) (4). Here, PRTN3 is linked to rheumatoid arthritis.