Delivery Disruptions

For gene therapy to be successful, literally billions of replacement genes need to be delivered, and they need to be delivered to just the right place. For example, if you're trying to cure a disease in the liver, you don't want genes winding up in the bloodstream, you've missed the target. Scientists are still working on a way to guarantee the genes get exactly where they need to go.

Another obstacle is controlling how the gene functions. Even if the genes get to their target,
"different genes function at different intensities, much like a light on a dimmer switch. Scientists have yet to figure out how much to turn a gene off or on. Too little … and there's no benefit to the patient. Too much … and there can be side effects."*

The final obstacle is rejection. Just like a body can reject an organ transplant, a patient's immune system may attack the new genes, "rejecting" them. If the genes are rejected, they have no opportunity to have a lasting effect.

http://abcnews.go.com/onair/WorldNewsTonight/wnt_991208_CL_GeneTherapy_feature.html