Herpesviruses are a large (nearly 100 members) and diverse group of viruses found widely throughout vertebrates (for a review, see Roizman, 1996). Eight herpesviruses have been iso- lated so far from humans: herpes simplex virus 1 (HSV-1), herpes simplex virus 2 (HSV-2), hu- man cytomegalovirus (hCMV), varicella zoster virus (VZV), Epstein-Barr virus (EBV), human herpesviruses 6 and 7 (HHV6 and HHV 7), and Kaposi sarcoma-associated herpes virus (KSHV). EBV and KSHV are believed to contribute to two different human cancers (see Chap- ter 12). Herpesviruses are enveloped viruses with very large DNA genomes ranging in size from
120 to 230 kb. Herpesviruses can establish two types of infections, lytic and latent. Lytic infec- tions result in death of the host cell with production of virus. In a latent infection, the virus be- comes dormant. The viral DNA is maintained in the host cell nucleus, as a circular plasmid, but few if any viral genes are expressed. Latent infections can become reactivated leading to the production of virus and sometimes pathological consequences for the host. Several animal herpesviruses are associated with tumor formation. These include the Ranid herpesvirus 1 (RaHV-1), which can cause renal adenocarcinomas in leopard frogs (Davison et al., 1999), and Marek’s disease herpesvirus (MDV), which causes a T-cell lymphoma in chickens (for a review, see Ross, 1999). The mechanism by which these viruses cause cancer is not well understood.