Almost all papillomaviruses encode an E7 protein, but they are best studied in the oncogenic HPVs. The E7 proteins induce cell proliferation, allowing the virus to amplify its DNA in differentiating cells. E7 binds a multitude of host proteins such as pRB and the related pocket proteins, P130 and p107, cyclin dependent kinase inhibitors p21 and p27, histone acetyltransferase p300 and associated protein p600. E7 is approximately 100 amino acids in length and consists of two conserved regions (CR1 and CR2) with sequence similarity to adenovirus E1a and SV40 T antigen. CR2 contains a motif, LXCXE, important for binding to pRB. The C-terminal domain forms a zinc-binding domain that has some similarity to the metal binding domains of E6 and it is though that they probably arose from a common ancestor.