The concept of human gene therapy came on the heels of fundamental discoveries on the nature and working of the gene. However, realistic prospects to correct the underlying cause of recessive genetic disorders through the transfer of wild-type alleles of defective genes had to wait for the arrival of recombinant DNA technology. These techniques permitted the isolation and insertion of genes into the first recombinant delivery systems. The realization that viruses are natural gene carriers provided inspiration for gene therapy and, as engineered vectors, viruses became prominent gene delivery vehicles. Nonetheless, when put in the context of human and non-human primate studies, all vectors fell short of success regardless of their viral or non-viral origin. Recognition of issues such as inefficient gene transfer and short-lived or scant expression in the relevant cell type(s) prompted researchers to refine and develop several gene delivery systems, in particular those based on retroviruses, adeno-associated viruses and adenoviruses. Concomitantly, available technology was deployed to tackle disorders that require few genetically corrected cells to attain therapy. BioEssays 27: 506-517, 2005. (c) 2005 Wiley periodicals, Inc.
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