In 1987, it was proposed that the risk of developing ACPA-positive RA is explained by the HLA-DRB1 locus, in particular by those alleles that carry the sequence QKRAA, QRRAA or RRRAA in positions 70–74 of the DRB1 chain, also known as the shared epitope (SE) motif [4, 5]. Here, PRTN3 is linked to rheumatoid arthritis.