Retinoblastoma is an extremely rare malignant tumor that develops in the nerve-rich layers that line the back of the eyes (retina). The retina is a thin layer of nerve cells that sense light and convert it into nerve signals, which are then relayed to brain through the optic nerve. Retinoblastoma occurs most commonly in children under the age of three. The most typical finding associated with retinoblastoma is the reflection of light off a tumor behind the lens of the eye, which causes the pupil to appear white, the so called "cat's eye reflex" (leukokoria). In addition, the eyes may be misaligned so that they appear crossed (strabismus). In some affected children, the eye(s) may become red and/or painful. The presence of a retinoblastoma may cause glaucoma, a condition marked by a rise in the pressure within the eyeball preventing the normal drainage of fluid from the eye and potentially causing characteristic damage to the optic nerve. Retinoblastoma may affect one eye (unilateral) or both eyes (bilateral). Bilateral forms of retinoblastoma are hereditary. In most cases, retinoblastoma occurs spontaneously for no apparent reason (sporadic).