Papillomaviruses are small, unenveloped, double-stranded DNA viruses exhibiting cubic symmetry. About 55 nm in diameter, they cause epidermal papillomas and warts in a wide range of higher vertebrates. Different members of the group are generally species spe-cific. For example, bovine and human papillomaviruses infect only the hosts reflected in their names. In some cases, tumors caused by these agents can become malignant and the role of these agents as causes of certain human cancers is being clarified. Papillo-maviruses have not been grown in tissue culture, and most of the virologic information has derived from molecular studies.
The genomes of many of the papillomaviruses have now been cloned and compared by restriction endonuclease and DNA homology procedures. These studies have shown a wide genomic diversity among papillomaviruses that infect different species and also among those that infect humans. This has led to the allocation of numbers for the different genotypes.
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