HLA-DRB1 and rheumatoid arthritis: In support of these RA-specific predictions, the strongest source of RA risk comes from the HLA-DRB1 “shared epitope” motif[28] aa-70[(Gln/Arg)-(Lys/Arg)-Arg-Ala-Ala]aa-74, composed almost entirely of predicted RA risk residues.