Progressive retinal atrophy, or PRA as it is frequently termed, is a long recognized, hereditary, blinding disorder. The first modern description of this problem was in Gordon Setters in Europe, in the early years of the twentieth century, but since then PRA has been recognized in most purebred dogs. PRA is a disease of the retina. This tissue, located inside the back of the eye, contains specialized cells called photoreceptors that absorb the light focused on them by the eye's lens, and converts that light, through a series of chemical reactions into electrical nerve signals. The nerve signals from the retina are passed by the optic nerve to the brain where they are perceived as vision. The retinal photoreceptors are specialized into rods, for vision in dimlight (night vision), and cones for vision in bright light (day and color vision). PRA usually affects the rods initially, and then cones in later stages of the disease. In human families the diseases equivalent to PRA (in dogs) are termed retinitis pigmentosa.

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