Major histocompatibility complex (MHC) class 1 molecules bind to antigens for presentation on the surface of cells. The proteasome is responsible for producing these antigens from the components of foreign pathogens. MHC class 1 molecules consist of an a heavy chain that contains three subdomains (?1, ?2, ?3), and a non-covalent associating light chain, known as ?-2-Microglobulin. ?-2-Microglobulin associates with the a3 subdomain of the a heavy chain and forms an immunoglobulin domain-like structure that mediates proper folding and expression of MHC class 1 molecules. The ?1 and ?2 domains of the a heavy chain form the peptide antigen-binding cleft.