Adenoviruses: these viruses carry their genetic material in
the form of double-stranded DNA. When these viruses infect a
host cell, they introduce their DNA molecule into the host. The
DNA molecule is left free in the nucleus and does not
incorporate with host genome, and the instructions in this
extra DNA molecule are transcribed just like any other gene.
This vector system has shown real promise in treating cancer
and indeed the first gene therapy product to be licensed to
treat cancer is an adenovirus