Peptide binding to a given class I MHC molecule requires the presence of anchor residues that complement the physicochemical characteristics of the specificity pockets [36] . Anchor residues are usually found at peptide position 2 (P2), where they interact with pocket B, but sometimes at position 5 or 6 (P5 or P6), where they interact with pocket C or E. A C-terminal residue (PC) also typically binds in pocket F [8] . The anchor residues Glu or Asp (P2) in B4 have an electrostatically favourable interaction with the side-chain of the pocket B residue Aspa9, which points up from the b-sheet. Similarly, anchor residues P2 Arg in B15 and B19 provide an electrostatically favourable interaction with the negatively-charged residues Aspa34 and Glua62, the side-chains of which point towards the binding groove from the a-helix. The presence of Glu8 or Asp8 as major residues in the C-terminal position of the B4 motif is unprecedented, and there are no previous mammalian examples of negatively-charged residues in the C-terminal anchor position [16] . It is possible that the positively-charged Arga80 residue of the F pocket is the key to the binding of anchor residues P8Glu or P8Asp. There is an anchor residue in the central cavity region corresponding to pocket C in B12 and B4. This is very similar to the mammalian MHC class I molecules H-2 Kb and H-2 Db, and it may be related to the Glya68(69) residue without a side-chain located in the a-helix (the residue number in parentheses is derived from mammals, whereas that without parentheses is from the chicken). The volumes of the binding grooves in B4 and B12 are 662.45 Å 3 and 637.677 Å 3 , respectively, which are greater than those of B15 (582.595 Å 3 ) and B19 (581.676 Å 3 ) (Fig. 5) . This may explain why the former have anchor residues in the middle region of their binding peptides. In general, anchor residues point downwards into the binding groove, which allows them to fill the pocket and maintain the stability of the peptide-MHC complex.