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HLA class II histocompatibility antigen, DRB1 beta chain is a protein that in humans is encoded by the HLA-DRB1 gene. 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. The protein encoded by this gene belongs to the HLA class II beta chain paralogues. The class II molecule is a heterodimer consisting of an alpha （DRA） and a beta chain （DRB）, both anchored in the membrane. It plays a central role in the immune system by presenting peptides derived from extracellular proteins to T helper cells. Class II molecules are constitutively expressed in professional antigen-presenting cells （APC: B lymphocytes, dendritic cells, macrophages）, and could be induced in non-professional APCs. Mainly used for leukemia immunophenotyping, determination of primitive cell and monocyte differentiation.