All viral ORFs are encoded on one DNA strand (see figure). This represents a dramatic difference between papillomaviruses and polyomaviruses, since the latter virus type expresses its early and late genes by bi-directional transcription of both DNA strands. This difference was a major factor in establishment of the consensus that papillomaviruses and polyomaviruses probably never shared a common ancestor, despite the striking similarities in the structures of their virions.