HLA class II histocompatibility antigen, DRB1 beta chain is a protein that in humans is encoded by the HLA-DRB1 gene.[5] DRB1 encodes the most prevalent beta subunit of HLA-DR. DRB1 alleles, especially those encoding amino acid sequence changes at positions 11 and 13, are associated risk of rheumatoid arthritis.[6][7]