HLA class I and II molecules are hetero-dimers, composed of alpha- and beta- chains (https://en.wikipedia.org/wiki/Human_leukocyte_antigen#Classification ). In the case of class I molecules, beta-chain (i.e. beta-2 microglobulin) is fixed while alpha-chain is variable. Hence, class I molecules are named based on their alpha-chains. In contrast, both alpha and beta-chains of class II molecules can vary. Thus, names of the two chains are needed to specify a class II molecules (e.g. HLA-DPA1*01:03/HLA-DPB1*02:01). For DR locus however, alpha chains are not variable. Hence, names for DR molecules use only those of the beta-chain (e.g. HLA-DRB1*01:01). For HLA molecules, we use the nomenclature as described here (http://hla.alleles.org/announcement.html ).