CitBiP-derived epitopes were identified by epitope mapping and one of them exhibited cross-reactivity to cyclic citrullinated peptide (CCP). A. The serum levels of antibodies to cit-BiP-derived citrullinated peptides. The serum samples were taken from anti-citBiP antibody-positive rheumatoid arthritis (RA) patients (n = 18, closed circle) and healthy controls (n = 5, open circle). B. Comparison between the serum levels of anti-native peptide antibodies (open circles) and citrullinated peptide antibodies (closed circles). (RA, n = 18; healthy controls, n = 5) **: P < 0.01, ***: P < 0.001. C. Competitive assay of anti-CCP antibodies from RA patients with a BiP-derived citrullinated peptide. The horizontal lines indicate medians. P-values were calculated by Mann-Whitney U test.