Illustration of apolipoprotein-E4 molecules (Apo-E4 blue) in a blood vessel with red blood cells (red). 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.