PRTN3 and rheumatoid arthritis: Most RA patients are affected by the so-called seropositive form of RA, defined by the presence of specific antibodies: IgM rheumatoid factor (IgM-RF) and antibodies against citrullinated proteins (ACPA).5 Increased serum levels of these antibodies in healthy individuals imply an elevated risk of developing RA.6 Interestingly though, not everyone having increased IgM-RF or ACPA serum levels develops RA.