The papillomaviruses had been originally lumped together
with the polyomaviruses in one family, the Papovaviridae.
This was based on similar, nonenveloped
capsids and the common circular double-stranded DNA
genomes. As it was later recognized that the two virus
groups have different genome sizes, completely different
genome organizations, and no major nucleotide or amino
acid sequence similarities, they are now officially recognized
by the International Committee on the Taxonomy of
Viruses (ICTV) as two separate families, Papillomaviridae
and Polyomaviridae. As one considers the lack of any
overall homology among the viral genomes in the two
families, one should take note, however, of a helicase
motif of the PV E1 protein, a domain stretching greater
than about 230 amino acids, which has some sequence
similarity with the SV40 T-antigen, the parvovirus NS1
protein, and a planarian virus-like element (Rebrikov et al.,
2002). While there is no doubt that the respective helicase domains of these viruses are homologous, there is no
evidence that this establishes a monophyletic origin of
these four different groups of viruses.