Millions of people worldwide suffer from diseases of the retina that cause partial or complete blindness. Those with hereditary retinal degenerative disease suffer from a progressive loss of the light-sensing photoreceptor cells, caused by any of over 250 genetic mutations. The retina is the thin layer of tissue at the back of the eye that detects light and color. It contains photoreceptor cells, known as rods and cones, which are responsible for detecting light and converting it into electrical signals. The retina also contains nerves that relay the electrical signals to the brain, telling it what the photoreceptors are "seeing." These relay cells are known as bipolar cells and ganglion cells. Credit: Ernesto del Aguila III, NHGRI.