is a better way to treat rare genetic diseases; cure a single gene defect by introducing a 'correct' gene. The first gene-therapy
trials were conducted using patients with rare monogenetic disorders, but these are now outnumbered by the clinical testing of gene therapeutics for more common conditions, such as cancer, AIDS and cardiovascular disease. This is partly due to a failure to achieve long-term gene expression with early vector systems, a critical requirement for correcting many inborn genetic defects. Now, with the advent of adeno-associated viral (AAV) and lentiviral vectors, which demonstrate persistent gene expression in animal studies, this technological barrier may have been overcome. These vectors are anticipated to form the basis of many genetherapy?protocols for inherited genetic diseases.