In contrast, exhaustion occurs when established effector CD8+ T cells are chronically stimulated, and, unlike peripheral tolerance, the first ∼5 days of differentiation are largely indistinguishable between exhausted and fully functional effector CD8+ T cells (142–145), although recent evidence does suggest some degree of commitment to the exhausted state by day 5 post-infection in chronic infection models (146). Here, CD8A is linked to infection.