A gene defect results in the production of an abnormal protein balance in the retina, which is destructive of retinal cells. Gene therapy is aimed at counteracting the gene defect by substituting normal gene material at the site of the problem. For this purpose a courier (or 'vector') which is both reliable and safe is needed. The challenges being currently undertaken by researchers are to harness a suitable virus capable of acting as a courier of genetic material and for the substitute gene material then to be capable of being effective for a human lifetime. The prospective role of gene therapy will be to stop further retinal degeneration rather than to restore what sight has been lost.