Since anti-citrullinated protein/peptide antibodies (ACPA) are very specific for RA (specificity of 98%, sensitivity 68%) [16] and high amounts of citrullinated proteins, like fibrinogen, have been detected in the joint of RA patients, it is likely that some antigens in IC of RA patients are citrullinated. This evidence concerns the gene PRTN3 and rheumatoid arthritis.