To verify that the RA antibody response is specific for the citrullinated peptides (ACPA-specific) rather than the Arg-containing control peptides (EBV-specific), the reactivity of RA sera and HD sera to citrullinated linear and cyclic peptides and the corresponding Arg-containing (non-citrullinated) peptides, presented by a C-terminal biotin, were analysed by streptavidin capture ELISA. The gene discussed is PRTN3; the disease is rheumatoid arthritis.