Three types of IgG fractions were purified and subjected to liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MSI) analysis: serum ACPA purified by the binding ability to CCP from ACPA+ RA patients (ACPA/RA), total serum IgG deprived of ACPA from the same RA patients (FT/RA), and total serum IgG from healthy donors (total/HD) (Fig. 1c; Supplementary Fig. 1). Here, PRTN3 is linked to rheumatoid arthritis.