cell as a pharmaceutical agent to treat disease. Most commonly it is carried out using a vector, which transports the nucleic acid (usually DNA encoding a therapeutic gene) into the target cell. A vector can transduce a desired copy of a gene into a specific location to be expressed as required. Alternatively, a transgene can be inserted to deliberately disrupt an unwanted or mutated gene, preventing transcription and translation of the faulty gene products to avoid a disease phenotype. Gene therapy in patients is typically carried out on somatic cells in order to treat conditions such as some leukaemias and vascular diseases. Human germline gene therapy in contrast is restricted to in vitro experiments in some countries, whilst others prohibited it entirely, including Australia, Canada, Germany and Switzerland. Whilst the National Institutes of Health in the US does not currently allow in utero germline gene transfer clinical trials, in vitro trials are permitted. The NIH guidelines state that further studies are required regarding the safety of gene transfer protocols before in utero research is considered, requiring current studies to provide demonstrable efficacy of the techniques in the laboratory. Research of this sort is currently using non-viable embryos to investigate the efficacy of germline gene therapy in treatment of disorders such as inherited mitochondrial diseases. Gene transfer to cells is usually by vector delivery. Vectors are typically divided into two classes – viral and non-viral. Viral vectors Viruses infect cells by transducing their genetic material into a host's cell, using the host's cellular machinery to generate viral proteins needed for replication and proliferation. By modifying viruses and loading them with the therapeutic DNA or RNA of interest, it is possible to use these as a vector to provide delivery of the desired gene into the cell. Retroviruses are some of the most commonly used viral vectors, as they not only introduce their genetic material into the host cell, but also copy it into the host's genome. In the context of gene therapy, this allows permanent integration of the gene of interest into the patient's own DNA, providing longer lasting effects. Viral vectors work efficiently and are mostly safe but present with some complications, contributing to the stringency of regulation on gene therapy. Despite partial inactivation of viral vectors in gene therapy research, they can still be immunogenic and elicit an immune response. This can impede viral delivery of the gene of