Formation of cryptic CTL epitopes by cleavage of amino-terminal residues in a peptide vaccine. Peptides NY-ESO-1_157-165 and NY-ESO-1_157-167, representing HLA-A2–restricted CTL epitopes, were used to vaccinate cancer patients. By the action of an aminopeptidase (P), peptide NY-ESO-1_157-167 can generate the truncated peptides NY-ESO-1_158-167 and NY-ESO-1_159-167, which are also capable of binding to HLA-A2 molecules and were shown to induce low-quality CTLs that failed to recognize tumor cells. Anchor residues, within the peptides, that facilitate MHC binding are shown in boldface. In the case of NY-ESO-1_157-165, the cysteine (C) anchor is likely to be suboptimal, since it does not represent a canonical HLA-A2–binding motif.