Fibrinogen is a protein produced by the liver which helps stop bleeding by helping blood clots to form. Fibrinogen gets deiminated (conversion from arginin to citrullin) by Peptidyl Arginine Deiminase (PAD) in inflamed joints in patients that develop rheumatoid arthritis.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. Proteins that normally contain citrulline residues include myelin basic protein (MBP), filaggrin, and several histone proteins, while other proteins, like fibrin and vimentin can get citrullinated during cell death and tissue inflammation. Patients with rheumatoid arthritis often (at least 80 % of them) develop an immune response against proteins containing citrulline. Although the origin of this immune response is not known, detection of antibodies reactive with citrulline containing proteins or peptides is now becoming an important help in the diagnosis of rheumatoid arthritis.Synonyms: FGA, FGB, FGG