Whereas naïve CD8 T cell repertoire diversity in the mouse declines to the extent that the ability to respond to new infections is impaired [3–6, 13, 22, 23] and “holes” in the repertoire can develop [7], it has been shown that there is only a modest (3–5-fold) reduction in CD8 T cell repertoire diversity with age in humans [60, 61]. This evidence concerns the gene CD8A and infection.