HLA-DRB1 and rheumatoid arthritis: Residue 74 is part of the ‘shared epitope’, a five-amino-acid sequence located at the centre of the peptide binding groove, previously associated with rheumatoid arthritis (RA).39 Despite extensive studies, the mechanistic role of the shared epitope in RA remains elusive.40 At residue 86, a valine is present in HLA-DRB1*04:04, as compared with the reference glycine (present in, for example, HLA-DRB1*04:01), both non-polar amino acids that differ in the size of side chains.