HLA-DRB1 and rheumatoid arthritis: The determining factor for HLA-associated RA susceptibility is a positive charge at the DRβ71 amino acid residue—DRB1*04:01 and DRB1*04:04 have a positively charged lysine or arginine, respectively, at this position whereas DRB1*04:02 has a negatively charged glutamic acid (80).