Apolipoprotein E (ApoE) is a protein that combines with fats (lipids) to form lipoproteins. Lipoproteins package cholesterol and other fats and carry them in the bloodstream. Apolipoprotein E is a major component of a particular type of lipoprotein known as very low-density lipoproteins (VLDLs). VLDLs remove excess cholesterol from the blood and carry it to the liver for processing. It has long been known that maintaining normal levels of cholesterol is essential for the prevention of disorders that affect the heart and blood vessels (cardiovascular diseases), including heart attack and stroke. But more recent research has revealed the numerous other roles played by ApoE in the body, and thus numerous other ways in which it can cause disease. Of particular concern for researchers is the mechanism by which different variants of the protein (ApoE2, E3, or E4) cause or hinder the development of dementia.