Three previously noted features of HLA-B27, which distinguish it from other heavy chains, underlie the hypothesis : (1) HLA-B27 has been found to be bound to peptides longer than 9-mers, suggesting that the cleft can accommodate a longer polypeptide chain; (2) HLA-B27 has been found to itself contain a sequence that has also been actually discovered to be bound to HLA-B27, as an independent peptide; and (3) HLA-B27 heavy chains lacking β2 microglobulin have been seen on cell surfaces.