IgG autoantibodies to citrullinated proteins (ACPA) precede the development of rheumatoid arthritis (RA). HLA-DR alleles with a 5-amino acid stretch called “shared epitope (SE)” in their P4 pocket are associated with ACPA and RA. The recent article by Ting et al. (1) suggests that the basis for this association is preferential binding of citrullinated peptides by SE-positive HLA-DR molecules. It shows binding of 13 selected peptides (3 from vimentin and 2 from fibrinogen) and their 18 citrullinated variants to 3 SE-positive alleles and presents 8 crystal structures of 3 SE-positive HLA-DR alleles complexed with a citrullinated peptide.