Gene therapy is a potential treatment regimen where exogenous genes or genetically engineered cells are introduced into individuals to relieve a disease phenotype. A number of methods for the delivery of DNA or RNA are under active investigation. The goal of therapy is to produce therapeutic proteins or alter gene expression in the host to stabilize or correct a potential disease state by providing appropriate intracellular homeostasis or by reducing the concentration of extracellular toxins. Difficulties with the cellspecific targeting of vectors, cell-specific transgene expression and regulation, the toxicity of delivery vehicles, and host immune responses have all hampered the achievement of these goals. To date, there has been limited success with human gene therapy for genetic disease and no successful therapy involving the central nervous system (CNS) (see http://www4.od.nih. gov/oba/rdna/htm for past and current gene therapy trials).