Inherited forms of retinal degeneration, which afflict 1 in 3000 people worldwide, arise primarily from mutations in cells of the eye’s outermost retinal layer (Fig. 1). These include photoreceptor cells—light-detecting neurons in the retina of vertebrate eyes or cells of the retinal pigment epithelium (RPE) a layer of pigmented cells that lies just outside of and supports the photoreceptors. The outer retina is, therefore, the primary target for ocular gene therapies (1), which deliver a wild-type copy of the mutated gene to the appropriate cells (transduction) typically by using an adeno-associated virus (AAV) vector.