Numerous diseases of the nervous system result from single gene or multifactorial gene defects such as cancer, immune pathological disorders, metabolic diseases, and common neurodegenerative syndromes (Parkinson’s and Alzheimer’s diseases). A greater understanding of the molecular, biochemical, and genetic factors involved in the progression of a specific disease state has led to the development of genetic therapies using direct gene transfer to ameliorate the disease condition or correct a genetic defect in situ. Standard gene therapeutic approaches employing retroviruses have not proven feasible for treating disorders of the central nervous system (CNS) since these vectors require dividing cells for integration and expression of the transgene, whereas CNS neurons are postmitotic, terminally differentiated cells. Thus, methods for delivery and expression of therapeutic gene products to treat CNS disease will require new delivery strategies and vehicles including the development of novel vectors for direct gene transfer. These vectors should: efficiently deliver the therapeutic gene(s) to a sufficient number of nondividing neurons; persist long-term in a nonintegrated state within the nerve cell nucleus without disturbing host cell functions; and be able to regulate therapeutic gene expression for diseases that may either require high-level transient transgene expression or continuous low level synthesis of the therapeutic product.