These helped to determine the functional and structural integrity of the fovea, and the area of preserved photoreceptors (ellipsoid zone on OCT) and RPE cells (AF ‘islands’) to which gene therapy could be applied, since the AAV vector is designed to replace the non-functioning CHM gene within surviving cells but would not be expected to rescue areas where the cells have already been lost.12 OCT was also used to rule out cystoid macular oedema, retinoschisis or choroidal neovascularisation at baseline, which could occasionally associate with CHM. This evidence concerns the gene CHM and choroideremia.