Clearance of the virus from the CNS in resistant haplotypes is dependent on a class I restricted CD8+ cytotoxic T lymphocyte (CTL) response, because normally resistant mice that are depleted of CD8+ T cells or lack the expression of beta 2 microglobulin (b2M) become susceptible to persistent viral infection [39–41]. The gene discussed is CD8A; the disease is viral infectious disease.