PRTN3 and rheumatoid arthritis: The reactivity toward cCK13‐1 was lower than that toward other ACPA antigens, such as cFIBβ, CEP‐1, cVIM, and cTNC5, but was citrulline‐specific as no enhanced antibody reactivity against the arginine control peptide was detected in RA sera, and 99% of patients who were positive for cCK13‐1 were also positive for CCP‐2.