The concept is compellingly attractive: replace a faulty, disease-promulgating gene with a normal one and, voila, a cure should be initiated. This has been the hope for gene therapies for many years. Unfortunately, everything has not gone according to plan, and the technology fell out of favor until comparatively recently. Confidence is now returning and expectations are once again high that the gene therapy field can finally make good on its promise to transform 21st century medicine.