PRTN3 and rheumatoid arthritis: The citrulline specificity of the ACPA response in patients with RA was confirmed by the lack of correlation with the response to the arginine‐containing peptides (Figures 2 and 3) and by complete absorption of the anti–CEP‐1 response in 6 BR/RA serum specimens only with the CEP‐1 peptide (Figures 4C and D).