Extensive homology exists between Marek disease herpesvirus and its vaccine virus, herpesvirus of turkeys.

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Abstract
Marek disease is a lymphomatous disease of chickens caused by infection of a herpesvirus, Marek disease virus (MDV). Marek disease is the only neoplastic disease for which a successful vaccine has been developed. The vaccine virus, herpesvirus of turkeys (HVT), is non-oncogenic in chickens. Despite the strong antigenic relationship between these viruses, previous studies showed that the 2 viral DNA share little or no homology. Using less stringent hybridization conditions and methods that greatly improve the reassociation kinetics, the sequence homology between MDV and HVT DNA was reexamined. HVT and MDV are far more closely related than previously reported. The homology between these 2 viral DNA ranges between 70% and 80% at the nucleotide level and appears to extend over 90-95% of the respective genomes. Under the low stringency conditions used, MDV DNA fails to cross-hybridize with DNA from feline rhinotracheitis virus, an antigenically unrelated herpesvirus with a G-C content identical to that of MDV and HVT.