Anti-CarP are present in about 45% of RA patients as well as in 10–20% of patients previously considered ACPA-negative [107] and have also been found in patients with palindromic rheumatism (PR) but in a lower proportion and with a different isotype usage than in RA, suggesting a distinct B-cell response to homocitrullinated antigens in PR [113]. This evidence concerns the gene PRTN3 and rheumatoid arthritis.