Expressed class I HLA molecules are composed of two different chains non-covalently bound. Their alpha chains (45 kDa), containing the peptide binding groove, is encoded by class I MHC genes, while their non covalently bound beta chain or beta2microglobulin (12kDa) is encoded on the chromosome 15. Beta2microglobulin binding is necessary for class 1 MHC molecules expression and for their proper folding for the right conformation of the peptide-binding groove. Their peptide binding groove is closed at the ends and accomodates short peptide ranging from 8-11 amino acids long. This groove has certain pockets that are the actual regions binding the anchor side chains of antigenic peptides. Specific pockets within the groove determine the sets of different peptides able to bind by class I MHC molecules. The stoichiometry necessary for proper signaling for CD8 T cell activation is one class I HLA molecule to one T cell receptor molecule.