Herpes is a virus that no one wants to get, but it has certain properties that make it potentially useful in gene therapy: it is very good at targeting the nervous and immune systems and it is capable of carrying a large genetic payload - about 35KB, or about six genes. The problem is that it's neuropathic, which necessitates the removal of multiple genes to render it safe. That, in turn, leads to production difficulties. BioVex Ltd. believes it has solved these impediments.

The difficulty in producing HSV vectors involves the removal of essential toxic genes from the virus. Because those genes are necessary for viral production, they must be put into producer cell lines, where they end up killing the cells. In addition, their presence in the manufacturing process means there is always the danger that the genes may be reintegrated into the vector.