Infection of mice with the Armstrong strain of lymphocytic choriomeningitis virus (LCMVARM) leads to a robust immune response and efficient viral clearance. This is in contrast to infection with the variant strain LCMVClone13, which causes functional inactivation of effector T cells and viral persistence. The mechanism by which LCMVClone13 suppresses the antiviral immune response and persists in its host is unknown.