ALB and rheumatoid arthritis: Because the stoichiometry of citrullination of untreated albumin is low (except for R98), we systematically screened RA and HC sera by ELISA for IgG binding to 14 citrullination sites using 11 pairs of peptides, 9 to 14 amino acid residues in length, with or without citrulline (Fig. 4C).