Anti-CCP2 IgG contains autoantibodies directed to citrullinated proteins/peptides (ACPAs) of various fine specificities. Analysis was conducted by sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. SDS-PAGE was followed by Coomassie staining and Western blot of uncitrullinated and citrullinated: fibrinogen, mutated vimentin, and α-enolase (all at 1 μg/well). Coomassie staining (left), Western blot using a pool of 11 purified anti-CCP2 IgG eluate fractions from synovial fluid (center) or a pool of the corresponding CCP2 column IgG flow-through fractions (right) are shown. CCP, cyclic citrullinated peptide; IgG, immunoglobulin G.