Leber’s hereditary optic neuropathy, which usually affects young men and leads to progressive sight loss. It is caused by a mutation in one of the genes inside mitochondria, the energy-producing structures inside cells. This kills off cells of the retina, the patch of tissue at the back of the eye that turns light into electrical signals. The gene therapy involves injecting a harmless virus containing the gene into the eye, where it is taken up by retinal cells. These start making the protein encoded by this gene, which passes into their mitochondria and helps preserve their remaining retina. A recent trial was deemed unsuccessful because the virus can just leave the eye. It is said that the way the virus travels is through the optic nerve and eventually transports to the other optic nerve where both optic nerves meet.