Interestingly, despite the short half‐life of 5.9 days of IgM in RA [116], and the fact that long‐lived IgM‐secreting plasma cells are not described in humans, IgM ACPA continues to be present in the serum of RA patients, or can occur at later stages in previously IgM ACPA‐negative patients [117, 118. Here, PRTN3 is linked to rheumatoid arthritis.