Cell culture amplification of a defective Marek's disease virus

A highly amplified 4-kb EcoRI fragment was present in DNA isolated from high cell culture passaged stocks (>93 passages) of 281MI/1, a serotype 2 Marek's disease virus (MDV). The isolated 4-kb fragment is amplified in the presence of MDV, replicating as a high molecular weight, head-to-tail concatemer. When the 4-kb fragment was cloned into pUC18 and cotransfected with MDV DNA into chicken embryo fibroblast cells, the plasmid clone also replicated as a high molecular weight concatemer.