Examination of the crystal structures of the MHC class 1 (Ref. 6) and class| I (Ref. 7) has identified peptides lying in an extended conformation in a peptide-binding groove on~ he suNace of the MHC molecune iFig. 2)''. Some residues of the peptide point down into the peptide-binding groove and interact with the MHC molecules, while others project up from the groove and are thus accessible for recognition by'T-cell receptors. Analysis of peptide binding to MHC molecules has shown that some of the amino acid residues of a peptide are crucial for efficient binding to MHC molecules and are known as