Although we were unable to directly assess cytotoxic function in our study, the observed differences in perforin expression—particularly the impaired regulation in CD8+ perforinlow and NK perforinlow populations, as well as the reduced expression in CD8+ perforinhigh and NK perforinhigh populations in XMEN patients—suggest a likely reduction in cytolytic capacity. The gene discussed is PRF1; the disease is X-linked immunodeficiency with magnesium defect, Epstein-Barr virus infection and neoplasia.