The HLA-DRA gene at 6p21.3 is one of the HLA class II alpha chain paralogues, and it plays a central role in the immune system by presenting peptides derived from extracellular proteins. Unlike the alpha chains of other Human MHC class II molecules, the alpha subunit is practically invariable. The promoter for the HLA-DRA is the most well studied, and it contains sites, from 5' to 3', for the trimeric RFX, CREB, the trimeric NF-Y, Oct-1, and YY1. RFX, CREB, and NF-Y contribute to the binding of the CIITA co-activator, which is either constitutively expressed in cells that constitutively express the MHC class II genes, or is induced by IFN-gamma activation of STAT1alpha. The beta-2 domain of the conserved HLA-DR alpha molecule encoded by the HLA-DRA1 locus interacts with the CD4 molecule in human lymphocytes. Class II molecules are expressed in antigen presenting cells (APC: B lymphocytes, dendritic cells, macrophages).