Variable-sized free episomes of Shope papilloma virus DNA are present in all non-virus-producing neoplasms and integrated episomes are detected in some.

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Abstract
The state of rabbit (Shope) papillomavirus DNA in virus-induced nonproducing tumors on domestic rabbits was investigated. Virus-specific sequences were resolved into many distinct bands by 1-dimensional agarose gel electrophoresis. Two-dimensional gel electrophoresis, CsCl/propidium iodide density equilibrium centrifugation, partial digestion with a restriction endonuclease and S1 nuclease digestion permitted identification of the bands as free viral episomes representing circular molecules of increasing size. In some tumors (both papillomas and carcinomas), up to 25% of the virus-specific DNA was linear and comigrated with cellular DNA. Integration of at least some of these sequences was suggested by the detection of viral-cellular junction bands in 1 tumor after digestion of DNA with EcoRI and SalI, enzymes that cut Shope DNA once. The physical states of viral DNA in papillomas and carcinomas were found to be similar, although free episomes were generally larger in carcinomas.