Epstein-barr Virus

The Epstein-Barr virus (EBV), the first isolated human tumour virus, was identified in 1964 by Epstein’s group in a cell line derived from Burkitt lymphoma (Epstein et al., 1964). EBV is a human herpesvirus, classified within the gammaherpesviruses subfamily, and is the prototype of the Lymphocryptovirus genus. In keeping with the systematic nomenclature adopted for all human herpesviruses, the formal designation of EBV is human herpesvirus 4 (HHV-4). Two major EBV types have been detected in humans: EBV-1 and EBV-2 (also known as types A and B). EBV-1 and EBV-2 differ in the sequence of the genes that code for the EBV nuclear antigens (EBNA-2, EBNA-3A/3, EBNA-3B/4, and EBNA-3C/6) (Sample et al., 1990). EBV-2 immortalizes B cells less efficiently than EBV-1 in vitro, and the viability of EBV-2-infected lymphoblastoid cell lines is less than that of EBV-1-infected lines (Rickinson et al., 1987). The differences in the immortalizing efficiency of the EBV subtypes may relate to a divergence in the EBNA-2 sequences (Cohen et al., 1989). In addition to type-specific polymorphism, significant DNA-sequence heterogeneity has been found when comparing selected regions of the EBV genome isolated in certain geographic areas or even from the same area. These polymorphisms define different viral strains within both types (Aitken et al., 1994).