PRTN3 and rheumatoid arthritis: Citrullination plays an important pathogenic role in anti-citrullinated protein antibody (ACPA)-positive rheumatoid arthritis (RA) [2, 3] and, possibly, in a number of other autoimmune diseases, inflammatory diseases or neurodegenerative conditions, including multiple sclerosis [4], Alzheimer’s disease [5], psoriasis [6], Sjögren’s syndrome [7], type 1 diabetes [8] and chronic obstructive pulmonary disease [9].