A number of different viruses have been proposed as potential vectors
for gene delivery to the nervous system. Herpes simplexvirus (HSV) has
particular advantages in this regard since it establishes life long a
symptomatic infections of the nervous systems and has a large genome s
ize capable of accepting large amounts of foreign DNA. Two methods of
using HSV-based vectors in gene delivery have been proposed. These inv
olve either the insertion of the foreign gene directly into the virus
to create a recombinant virus or its insertion into an amplicon plasmi
d vector which contains an HSV origin of replication and packaging sig
nal and which requires a helper HSV virus in order to be propagated. I
n both cases, it is necessary to disable either the recombinant virus
itself or the helper virus to prevent damaging effects due to lytic re
plication by the virus whilst maintaining the efficiency of gene deliv
ery. This disabling process is one of the major challenges still to be
overcome before HSV-based vectors can be used in clinical therapy, th
e over being the development of means to ensure that the foreign gene
is expressed in the long term after the virus has entered latency. The
potential means of overcoming these problems and the potential use of
HSV-based vectors in a number of different situations are discussed.