HLA-DRB1 and rheumatoid arthritis: In order to take a mechanistic approach to elucidating the linkage between the SE, RA, PD, we chose to use a “humanized” mouse model in which I-A ̊ (mouse class II-null) C57BL/6 mice were provided with a transgene encoding a chimeric form of mouse/human HLA-DRβ1.