HLA-DRB1 and rheumatoid arthritis: Extensive genetic studies during the last 40 years have demonstrated substantial contribution from the human leukocyte antigen DR beta chain 1 shared epitope (HLA-DRB1 SE) alleles in RA pathogenesis, specifically for the subtype of RA that is positive for anti-citrullinated protein antibody (ACPA) [1–11].