Earlier analysis [21, 22] using sequential alanine substitutions in the immunogenic NP 366-374 peptide established that there is some cross-reactive, though diminished, stimulation following the incubation of wt D b NP 366 -specific CD8 + T cells with the NP mutant peptide containing a single asparagine to alanine substitution at position (P) 3 (NPN3A mutant). Such cross-reactive CD8 + T cell responses after NPN3A stimulation are not surprising as the partially solvent exposed P3N residue is unlikely to play any prominent role in contacting the TCR [23] . The wt NP 366-374 peptide binds to H-2D b in an extended conformation where the P2-Ser, P5-Asn and P9-Met are the anchor residues, P3-Asp is a semi-anchor (partially buried/partially exposed), and P4-Glu, P6-Met, P7-Glu and P8-Thr are solvent exposed and available for contact by the TCR (Fig. 1) . This provides the basis for a defined experimental system that can be used to determine what happens when a TCR repertoire is selected to what would seem (from in vitro analysis) to be a suboptimal, cross-reactive mutant epitope. The interpretation of in vitro analysis alone should, however, be tempered with caution, as another mutant (M6A) that did not cross-react at all with the wt epitope elicited a completely novel, in vivo endogenous CTL response of equivalent magnitude when expressed in an infectious influenza A virus [22] . What would be the case for TCR responses elicited by influenza A viruses expressing the mutant NPN3A peptide in the native viral protein?