Alzheimer disease is a medical condition in which abnormal substances build up in the brain, causing brain cells to gradually degenerate. As the brain tissue shrinks, patients experience a loss of mental abilities known as dementia. Apolipoproteins help transport fatty substances, or lipids, in the blood. Apolipoproteins and lipids form large particles called lipoproteins. There are several types of apolipoproteins. One particular apolipoprotein, apolipoprotein E (apoE) is thought to have something to do with Alzheimer disease. That is, people who carry a gene for a certain form of apoE (E4 isoform) may be more likely to develop Alzheimer disease. However, many people with apoE E4 never develop Alzheimer disease. Another apolipoprotein, apolipoprotein(a) [apo(a)], shares many common features with apoE, but its function in humans is unknown. However, because it resembles apoE so closely, experts believe it might also have some role in Alzheimer disease.