2 regulatable expression. These characteristics are dif- ficult to find in a single vector system, and hence, a variety of viral gene delivery systems have been developed, each with its own advantages and dis- advantages (Table 1). Herpes simplex virus Wild-type Herpes simplex virus (HSV) is a large double-stranded DNA virus that infects and replicates in the skin and mucous membranes. Due to the natural tropism of HSV, initial studies were performed on neurons (Ho and Mocarski 1988, 1989). Many studies have now demonstrated long-term stable transgene expression in the nervous system (Glorioso and Fink 2009), and preclinical studies on models of neurological disease, including glioma, peripheral neuropathy, chronic pain and neurodegeneration, show encouraging results (Cuchet et al. 2007). Wild- type HSV is taken up by sensory nerve terminals and transported by retrograde axonal transport to dorsal root ganglion neurons where they may enter the lytic cycle or establish a latent state from which the virus may subsequently reactivate and spread. The HSV vector systems therefore mimic this latent state, producing a highly infectious, efficient vehicle for the delivery of exogenous genetic material to cells. The straightforward production of high-titre, pure preparations of non-pathogenic HSV vectors, known as amplicon vectors (Cuchet et al. 2007), was achieved by introducing mutations into viral immediate early genes. This readily disrupts the Cell Biol Toxicol (2010) 26:1–20 capacity for viral replication, but enables production of the vectors by in vitro complementation of these genes in trans. When used to transduce cells, these HSV vectors cause a latent-like infection in both neural and non-neural tissue. Further details on HSV vecterology can be found in reviews by Epstein et al. (Cuchet et al. 2007; Epstein 2005). Adeno-associated virus Adeno-associated viruses (AAV) are parvoviruses that require co-infection with another helper virus (either adenovirus or HSV) to mediate their replication. The small non-enveloped icosahedral virions (18- to 26-nm diameters) contain a single-stranded DNA molecule (4–5 kb) that contains inverted terminal repeats (ITR) which enable site-specific integration of wild-type AAV into chromosome 19 as well as allow secondary structure formation of the viral genome that aids viral DNA replication with host cell polymerase (Ni et al. 1998). The non-pathogenic and persistent long-term nature of AAV infection (Wright et al. 2003), combined with its wide range of infectivity, made this virus an important candidate as a therapeutic gene transfer vector. In addition, the relatively small cis-acting elements (the ITRs) in AAV vectors greatly reduce the risk of recombination with wild-type virus as well as the level of cellular immune response elicited, therefore improving the safety of this vector in human clinical applications (Koczot et al. 1973). Furthermore, AAV vectors have a broad host and cell type tropism transducing both dividing and non-dividing cells. Table 1 Viral vector systems Vector Retrovirus (inc lentivirus) Adenovirus AAV HSV Entry Fusion Receptor Receptor Fusion Immune response Capacity (kb) Location Ease of production Expression Minimal Brisk*** None Brisk**** 8 36 4 30 Integrates Episomal Integrates Episomal 108** 1012***** 108*** 1010**** Years Weeks/months Years Weeks The main characteristics of the four main viral vector systems being used to transfer genetic material into cells both in vitro and in vivo are compared above. Each system has been modified since their initial creation, enabling the capacity for the foreign transgene to be increase, especially in the case of adenoviral vectors where gutless vectors have a very large cloning capacity. The host immune response differs between each system, with retroviruses and AAV showing little or negligible immune response. In addition, these two systems facilitate long-term transgene expression. Adenoviral vectors have also shown increased longevity of transgene expression, especially when the promoter included in the viral cassette is modified from CMV Asterisks indicate the relative ease of production or the relative level of immune response, i.e. a greater ease of production or greater immune response elicited is represented by a higher score