HLA class I proteins are expressed on the surface of all nucleated cells but varying degrees. They are comprised of one trans-membran heavy chain with three extra-cellular domain and ß-2 microglobulin light chain. Overall, a class I molecule has an immuno-globulin-like tertiary structure with the most extra-cellular domains being the location of nearly all amino acid allelic variability. The normal function of HLA class I proteins is presentation of peptides from expired or defective intracellular proteins and peptides from invasive viruses from within the cell to the T cell receptor on CD8. T cells (often cytotoxic) leading to immune mechanisms which destroy to the cell. HLA proteins also interact with killer inhibitory receptors expressed on the surface of natural killer cells, often leading to inhibition of natural killer cell activity.