A cataract is a clouding or opaque area over the lens of the eye — an area that is normally transparent. As this thickening occurs, it prevents light rays from passing through the lens and focusing on the retina — the light-sensitive tissue lining located in the back of the eye. With some cataracts, this clouding is caused when some of the protein which makes up the lens begins to clump together and interferes with vision. Cataracts occur in about 0.4% of children. They can affect either one eye (unilateral) or both eyes (bilateral). Some cataracts are small and do not cause any visual symptoms. However, other, more progressive, cataracts can cause visual problems in children. Cataracts in children are uncommon.