This is supported by the fact that IgM ACPA is found in sera of both early undifferentiated arthritis and established RA patients.101, 102 This observation is important, since the IgM half‐life is only 5‐8 days103 and IgM secreting plasma cells are known to be mostly short‐lived.104 Together, this suggests a continuous activation and recruitment of citrulline‐reactive B cells. This evidence concerns the gene PRTN3 and rheumatoid arthritis.