Illustration of apolipoprotein-E4 molecules (Apo-E4 blue). Apolipoproteins are proteins that bind lipids, such as fats or cholesterol, to form lipoproteins. Apo-E is involved particularly in the uptake of cholesterol and is the main apolipoprotein in the central nervous system. The E4 variant of the Apo-E gene is associated with many health issues including Alzheimer's disease, though the nature of this association is not well understood. In the background are healthy red blood cells (red) and white blood cells (translucent).