Modified citrulline

Citrulline, while being an amino acid, is not built into proteins during protein synthesis, as it is not coded for by DNA, yet several proteins are known to contain citrulline. These citrulline residues are generated by a family of enzymes called peptidylarginine deiminases (PADs), which convert arginine into citrulline in a process called citrullination or deimination.