The genetics of HLA-DR is complex. HLA-DR is encoded by several loci and several 'genes' of different function at each locus. The DR α-chain is encoded by the HLA-DRA . Unlike the other DR loci, functional variation in mature DRA gene products is absent. (Note: see table "Number of Variant Alleles HLA-DR Loci"- reduces the potential functional combinations from ~1400 to ~400 (is not exact because new alleles are continually being added; not all new alleles are functional variants of the mature subunits).