Class II HLA molecules, both the alpha and beta chains, are encoded by the respective genes on the chromosome 6. They are both bound non-covalently and both contribute to the conformational organization of the peptide binding groove. The peptide binding groove in class II HLA molecules is open at the ends and accomodates larger peptides than for class I HLA molecules, ranging from 15-30 amino acid long. Similar to class I HLA molecules, pockets in the groove of class II HLA molecules for anchor side chains of the antigenic peptides also stabilise the interaction, but their binding so specific peptide side chains is less stringen than for class I HLA molecules. The stoichiometry necessary for proper signalling for CD4 T cell activation consist of dimers of class II MHC molecules interacting with dimers of T cell receptor molecules.