Apolipoprotein E (apoE) is a major constituent of lipoproteins in the plasma and 
in the brain. There are three common apoE isoforms, termed E2, E3, and E4. By 
virtue of its ability to bind to lipoprotein receptors, apoE plays a key role in 
the metabolism of triglyceride-rich lipoproteins in the plasma. Homozygous 
carriers of apoE2 have an increased risk to develop type III 
hyperlipoproteinemia, whereas apoE4 is associated with elevated levels of 
low-density lipoprotein cholesterol. In the brain, apoE is associated with 
cholesterol-rich lipoproteins and is involved in the transport of cholesterol to 
neurons. The genetic polymorphism of apoE is among the strongest determinants of 
the risk and mean age of onset of Alzheimer's disease. The mechanism by which 
apoE isoforms differentially contribute to disease expression is not known.