Retinal ganglion cells (RGCs) are found in the innermost layer of the retina. They integrate information from photoreceptors, via the bipolar cells of the retina, and project into the brain. Some RGCs are themselves photoreceptive; these are believed to be involved in regulating circadian rhythm. RGC axons are the sole output neurons from the eye whose axons form the optic nerve, optic chiasm and optic tract. RGCs are an important model system in developmental neurobiological research, particularly in the areas of axon guidance, formation of topographic maps and neural plasticity.