Is the smaller D b NPN3A + CD8 + T cell response a consequence of diminished naïve CTL [33] ? We found (Fig 4A) similar naive CTLp counts for D b NPN3A (34.6613.08) and D b NP 366 (28.5611.0), indicating that the smaller response to D b NPN3A + CD8 + is not due to reduced number of naïve precursors. Furthermore, assessing the extent of Vb8.3 bias (the dominant Vb for the D b NP 366 + CD8 + set) within the naïve D b NPN3A + CD8 + population ( Fig 4B) showed that the extent of Vb8.3 usage (mean 13.2%64.3) (Fig. 4B ) was much the same as that determined previously [33] for naïve D b NP 366 + CD8 + CTLps (mean 17.1%67.4) However, sequencing the naïve D b NPN3A + Vb8.3 + CD8 + TCR CDR3b regions showed a clear difference from the comparable wt-specific set. The ''public TCR'' dominance characteristic of D b NP 366 + Vb8.3 + CD8 + T cells in both pre-immune [33] and immune [34] TCRb repertoires [34] was not a prominent feature of the D b NPN3A-specific TCR repertoire. These public TCRs were found in only one (SGGANTGQL and SGGGNTGQL) or two (SGGSNTGQL) of the 10 mice tested (Fig. 4C) . Thus, although the naïve CTLp frequencies are comparable for D b NP 366 + Vb8.3 + CD8 + and D b NPN3A + Vb8.3 + CD8 + T cells and there is some overlap of some cross-reactive TCRs, the two repertoires are far from identical. The roughly equivalent numbers of precursors specific for the wt D b NP 366 and mutant NPN3A peptides were unexpected considering the lower response after infection with the mutated virus. The lower magnitude of the NPN3A + CD8 + T cell response and narrower TCRb repertoire suggest that only a proportion of naïve NPN3A + CD8 + precursors are being recruited into the immune response or that, once recruited, these cells do not expand efficiently. Inefficient recruitment and/or expansion early after influenza infection, despite large naïve CTL precursor numbers, have been recently documented by our group as key determinants of subdominance for D b PB1-F2 62 and K b NS2 114 -specific responses [33] .