An orphan disease is defined in the United States as a medical condition affecting fewer than 200,000 individuals. Six hundred orphan diseases affect an estimated 25 million Americans, or 9% of the population. Eighty percent of these rare diseases are genetic. Virtually all inherited retinal dystrophies and degenerations—except for age-related macular degeneration (AMD), which is quite common—fit the definition of an orphan retinal disease. Although individually rare, orphan retinal diseases collectively represent a major cause of untreatable vision loss and blindness.

The functions of many of the genes discovered for orphan retinal diseases were previously unknown, and represent new pathways in cell biology. These rare diseases become models of disease mechanisms in pathophysiology, which lead to insights into more common diseases. Orphan diseases, thus, provide platforms for the development of new therapeutic strategies that can be applied to diseases that are more common.