Subsequent studies have complemented the SE hypothesis and determined that almost all of the RA risk conferred by the MHC region is explained by six amino acids in four HLA molecules: HLA-DRB1 (positions 11/13, 71 and 74), HLA-B (position 9), HLA-DPB1 (position 9), and HLA-A (position 77) (17, 29, 30). Here, HLA-DRB1 is linked to rheumatoid arthritis.