Although CD8+ T cells of infected mice have the capability to recognize the cyst-harboring cells and eliminate a majority of them through perforin-mediated mechanisms (3, 5, 23), small potions of the cysts are able to avoid their elimination by the T cells and persist in the presence of the anti-cyst T-cell immunity (3, 5, 23) through unknown mechanisms. Here, PRF1 is linked to cyst.