HLA-DRB1 and rheumatoid arthritis: The Shared Epitope (SE) is a conserved amino acid sequence at positions 70–74 in the third hypervariable region of the DRB1 chain shared between RA-associated alleles (e.g., DRB1*04:01 and DRB1*01:01) and contributes to the P4 peptide-binding pocket.