PRTN3 and rheumatoid arthritis: ACPA, presently used as biomarkers for RA, recognize an array of human citrullinated proteins (including filaggrin, fibrinogen, alpha-enolase, histones, and vimentin) [9], generated through post-translational modification of arginine to citrulline catalysed by the family of peptidylarginine deiminases (PADs).