HLA-DRB1 and rheumatoid arthritis: In order to assess the structural bases that underlie stronger T cell responses toward HLA-DRB1*04:01 in complex with the PTM peptide Cit26 compared to the native α-enolase epitope 26 in RA patients, we determined the crystal structures of HLA-DRB1*04:01 in complex peptides 26 and Cit26 at 2.6 and 2.0 Å resolution, respectively (Figure 5; Table 3).