There are two forms of gene therapy that I am aware of. One is the use of an engineered virus to edit DNA and the other is the use of TALENS to edit DNA.
The use of a Virus hold tremendous potential because the primary function of a virus is to hijack a cells machinery to replicate itself. An engineered virus could be used to target eye cells in Aniridia patients to introduce the necessary genetic material to allow the production of the missing protein the causes Aniridia. Of course there is one critical problem with using a virus for gene therapy – the immune system.
Primers on Virology
Links to related research
The other method for gene therapy is TALENS. I have found a great video and article to explain the process far better than I could. So here is the video and link.