The infection of cord blood T cells we observed in cell culture was transient but the EBV infection of T cells in the patients appears to persist, perhaps because of the host genetic background, such as the CD137 and PIK3CD deficiencies in patient 12, as proposed previously for the familial cases of T or NK cell infections described in this paper (18, 19). This evidence concerns the gene TNFRSF9 and Epstein-Barr virus infection.