Herpesviruses have been detected in a vast variety of vertebrate species and in at least one invertebrate. It is anticipated that the approx 120 different herpesviruses known today represents only a fraction of the number that actually exists (1 ). Each virus is closely associated with its main host species. This hostspecific occurrence indicates that the herpesviruses have evolved with their hosts over long periods of time. Interestingly, many herpesviruses seem to be entirely avirulent within their original hosts. In contrast, upon infection of foreign hosts, i.e., those who did not participate during the process of coevolution, dramatic, often lethal diseases may occur (2 ,3 ). However, many herpesviruses are associated with various degrees of disease in their original host. The potential of herpesviruses to infect a broad range of host cells and to either induce or distract immune reactions makes them interesting entities to study in the context of both, development of new vaccines and vectors for gene therapy (1 ).