Inherited photoreceptor degeneration (IPD): The human retina is a highly specialised tissue that enables the perception of light across a range of intensities and colours. It covers about 65% of the inner surface of the eye and contains three layers of cells: the outer nuclear layer (ONL) containing the cell bodies and nuclei of the light-sensitive rod and cone photoreceptors whose photopigment-containing outer segments form the photoreceptor layer; the inner nuclear layer (INL) containing bipolar, horizontal and amacrine cells; and the ganglion cell layer (GCL) from which the optic nerve arises. There are two layers of synaptic connections between these three layers: the photoreceptors synapse with second order neurons, mainly bi-polar cells, in the outer plexiform layer (OPL), while in t the bipolar cells form connections in the inner plexiform layer (IPL) with ganglion cells. The retinal pigment epithelium (RPE) lies directly behind the photoreceptor layer, is heavily pigmented to reduce scattering of light, and is essential for the nourishment, maintenance and metabolism of photoreceptors.