 The DR β-chain is encoded by 4 loci, however no more
than 3 functional loci are present in a single individual, and no more than two on a single chromosome.Sometimes an individual may only possess 2 copies of the same locus,
DRB1*.The HLA-DRB1 locus is ubiquitous and encodes a very large number of functionally variable gene products (HLA-DR1 to HLA-DR17).