The HLA-E protein is one of the most extensively studied HLA class Ib antigens and the least polymorphic one compared to other HLA class I molecules. In the human population, there have been reported just ten alleles encoding three different peptides. Only two of these alleles namely HLA-E*0101 and HLA-E*0103, are widely distributed (around 50% each). The proteins encoded by these alleles differ from each other in one amino acid at position 107. In HLA-E*0101, it is arginine, and in HLA-E*0103, it is glycine. The difference between these proteins manifest itself in surface expression levels, affinities to leader peptides and thermal stabilities of their complexes. The HLA-E molecule is a ligand for CD94/NKG2 receptors on natural killer cells and T cell receptor, so it plays a double role in both innate and adaptive immunity.